Light

Fact of light

Luminous = Object that give light or emit light of their own. E.g sun,tourch etc.

Non-luminous=not produce their own light but reflect light so we can see eg. Moon, book, table etc

Transparent=object that allow light to pass through it. Eg air, glass

Opaque=which does not allow light to pass through it. Eg wood, wall, iron etc

Translucent=semi transparent. not all light pass through it.eg some plastic,sun glasses, frost glass etc

The speed of light is maximum in vacuum.

Light Wave and frequency:

Frequency: Frequency is the number of occurrences of a repeating event per unit time. It is denoted by symbol ν(nu).

Wavelength: light waves or infrared (heat) waves make characteristic patterns as they travel through space. Each wave has a certain shape and length. The distance between peaks (high points) is called wavelength. it is denoted by symbol λ(lemma).

We can see light only between 400 nm to 700 nm wavelength and the frequency of this light is between 400 THz to 800 THz. Wavelength and frequency have opposite relation, if frequency is low the wavelength is high.  We cannot see wavelength below 400 nm and above 700 nm. The red light have highest wavelength i.e. 700 nm and lowest frequency i.e. 400 THz of visible light  and violet color have lowest wavelength i.e. 400 and highest frequency i.e. 800 THz of visible light. The seven colors we can see are violet, indigo, blue, green, yellow, orange, red (VIBGYOR)

Ultraviolet: that light that have wavelength below 400 nm is called ultraviolet.

infrared: that light who have wavelengths above 700 nm is called infrared.

There is a little difference between lenses and mirror. Mirror work on reflection and lens work on refraction.  Light travel fastest in vacuum,It slows down in dense medium.

Reflection

When a light fall on a surface it is return back it is called reflections. A highly polished surface reflect most of the light like mirror. Incident ray is that which touch the surface the point where incident ray strike is called point of incident. A straight line drawn perpendicular to the mirror through point of incident is called normal or perpendicular line. The angle which the incident ray make with the normal line is called angle of incident. The ray which bounces off is called reflected ray. The angle which reflected ray make with normal is called angle of reflection.

Law of reflection:

The angle of incidence is always equal to the angle of reflection.

The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.

      

→ Reflection on a Plane Mirror: The image formed by a plane mirror is virtual & erect(not inverted image).The image distance is equal to the object distance from the mirror. The image is laterally(literal mean sideways) inverted i.e., the right side of the object appears as the left side of its image & vice-versa.

 →    for a given incident ray, if the plane mirror is rotated through an angle ‘q’ then the reflected ray rotates through an angle of ‘2q’.

→ a person can see his full image in a mirror of half the height of person.

→ Two plane parallel mirrors

The images formed by two parallel mirrors depend on the angle of mirrors. Formula is number of images(n)=360/angle of mirrors(q).then subtract 1 from n. Let suppose mirrors are placed at 60 degree then the images formed are:

  n=360/60 → n= 6-1 → 5.

Use of plane mirror:

→ When we see a series of images in a thick plane mirror, out of these the second image is brightest.

→ Out of 26 alphabets (e.g., A, B, C etc.) only 11 alphabets show lateral symmetry(in mirror image left side will not become right) and the rest 15 alphabets have lateral inversion(left side become right side in mirror image.)

→ If the object is displaced by a distance ‘x’ towards or away from the mirror, then its image will be displaced by a distance ‘x’ in the same sense

→If the mirror moves by a distance ‘x’ towards or away from the object, its image will move by a distance ‘2x’ in the same sense

 → If the object moves with a velocity ‘v’ towards or away from the mirror, its image appears to move with a velocity ‘2v’ in the same sense. The same is true if the mirror moves.

Reflection on Spherical Mirrors: spherical mirrors are of two types

(i) Convex or Divergent Mirror: A convex mirror is a mirror that is curved or hollowed outward in the middle. Focal point is behind the mirror, so it has a negative focal length. Usually it makes virtual image but it can make real image. it is the only mirror who can produce both virtual and real image. Thicker in centre. Since rays incident parallel to the principal axis are diverged away the principal axis, hence, it is known as a divergent mirror. it make image is: diverging, virtual image, upright, image behind mirror, reduced in size.

      Convex mirror give wider view because it reduce the size objects. It give wider view so it is used as rear view mirror in car and bike etc. it diverge the light away so it is used in a street lamp to diverge light over large area.

(ii)Concave or converging Mirror: A concave mirror is a mirror that is curving or hollowed inward. Thicker in sides and thinner in middle. All rays incident parallel to its principal axis are convergent at its focus. the focal point is in front of mirror. image formed by this is real, upside down or inverted, image formed in front of mirror, image size decrease with the distance of object form mirror.

       It focus all the coming rays to one point, so it is used in headlight of bike car, searchlight, telescope, solar cooker etc to focus the light so that the light goes too long and person can see farther object clearly. A concave mirror is used by dentists and ENT specialists to focus light on teeth, eye and throat to examine these organs. It is also used in shaving mirror as it formed erect and magnified image.

the surface of the spoon curved inwards can be approximated to a concave mirror and the surface of the spoon bulged outwards can be approximated to a convex mirror.

Types of images:

Real image=It is always inverted and in front of the concave mirror.it is upside down.

Virtual image= It is always erect and behind the concave and convex mirror.

Some important points:

Pole(P)= point at the side of mirror.

Centre of curvature(C)=centre of spherical mirror also called 2f

focus(f)=focus point is mid point between pole and centre of curvature. Every reflected ray of spherical mirror meet at a point, that point is called focus point. it depend on how curved the mirror is.

Focal length= distance between pole and focus is called focal length.

Principal axis=straight line from pole to centre of curvature through focus point. principal axis is a line that is perpendicular to the center of the mirror.

Radius of Curvature= distance between centre of curvature and pole.

Spherical Aberration: Inability of a concave lenses or mirror to converged parallel rays at a focus point.It can be removed by using a parabolic concave mirror of larger aperture

Important fact about mirrors.

Size of mirror does not affect the nature of the image except that a bigger mirror forms a brighter image. A virtual image cannot be taken on screen. But our eye lens forms a real image of the virtual image (acting as virtual objects) on the retina. A virtual image can be photographed.

-When we read a book, light is scattered by the paper into our eyes. This is called Diffuse-reflection. A printed paper has

a rough surface and hence, it cannot produce a regular reflection. That is why we read a book because of reflection of light but we do not see even a faint image of ours in its printed pages.

-The focal length of a plane mirror is infinity

-When a spherical mirror (concave or convex) is placed in a liquid e.g. water, its focal length does not change because

Focal length of a mirror depends upon its construction and not on the external medium.

note:  But if we talk about lens the focal length changes in water.

Refraction

When light goes from rarer (e.g. air) to a denser (e.g. glass) medium, it bends towards the normal to the interface, separating these two media. On the other hand, when light goes from a denser to rarer medium, it bends away from the

Normal to the interface separating these two media. This phenomenon is known as refraction of light.

        The cause of bending of light is when light goes to denser part the wavelength of light is changed, due to change in wavelength the speed of light decrease so it bend.

-the bottom of a tank or a pond containing water appears to be raised

-when a thick glass slab is placed over some printed matter, the letters appear raised when viewed through the glass slab

-pencil partly immersed in water in a glass tumbler.

-lemon kept in water in a glass tumbler appears to be bigger than its actual size, when viewed from the sides

Snell’s law:

                      

Here n is refractive index.

This equation is used to find the angle of refraction. We know refractive index(n) of air, water, glass etc. so it is easy to find the refractive angle

Principle of Reversibility: ‘If the path of a ray of light is reversed after suffering a number of refractions idth: and reflections, then in the given figure a light ray suffers refraction thrice at point B, C & D. It follows path AB → BC → CD → DE. If the final ray DE is incident at 90° to a plane mirror M, it retraces the original path as ED → DC → CB → BA.

Effects of reflection:

-Stars twinkle due to refraction. The different layers of atmosphere are of different (a) density, (b) temperature(c) speed and their density, temperature speed change continuously. Hence, the apparent position of the star changes continuously. It leads to the twinkling of a star

-The sun is visible to us before actual sunrise & after actual sunset. The atmosphere has higher density than that of outer space. Hence, light coming from the sun enters outer space to atmosphere i.e., from rarer to denser medium. It results in the bending of light. When the sun is below the horizon before the sunrise and after the sunset, its apparent position is visible above the horizon. Hence, the sun is visible 2 minutes before the actual sunrise and 2 minutes after the actual sun-set. Thus, the day becomes longer by 4 minutes due to effect of refraction.

lens:

There two types of lens concave lens and convex lens. Lens work on the principle of refraction.

Concave lens:

This lens is thin in middle and thicker in edges. The image formed is virtual, smaller than the object.

It diverge the rays of light outward because it is dense at edges and the light rays coming, bend toward denser part, so it diverge the light outward. This lenses is used in myopia(Short-sightedness).it power is negative

Convex lens:

This lens is thicker in middle and thin in edges. they are converging lens. It formed bigger images, real or virtual depend upon the position of object .our eyes have this type of lens.it is used in eye defect of Hypermetropia ( Long sightedness).Its power is positive.

Power of a Lens (P)

1/focal length. Focal length is measured in meters (m).

Lens power measured in diopter or Its S.I. unit is Dioptre (D). ID = 1m–1.

Smaller the focal length of a lens higher is its power.

Fect of lens:

-If a convex and a concave lens of the same focal length are put in contact the combination acts as a glass slab.

-If a convex and a concave lens of the same focal length are placed co-axially with a small separation between them, the combination acts as a convex lens

-If a lens is cut into two equal halves with a plane perpendicular to the principal axis, the focal length of each part doubles

-If a lens is cut into two equal halves with a plane along the principal axis, the focal length of each part remains unchanged

-An air bubble in water behaves as a concave lens

-When a lens is placed in a medium of higher density (higher refractive index), the nature of the lens reverses i.e. a converging lens behaves as a diverging lens & vice-versa.

-When a lens is placed in a medium of equal density, it acts as a glass slab. Its focal length becomes infinity and power becomes zero.

-the focal length of plane mirror is infinity.

Total internal reflection

the complete reflection of a light ray reaching an interface with a less dense medium when the angle of incidence exceeds the critical angle.

When a ray goes from denser to rarer medium it is refracted and bend away from normal. The angel refracted ray is always greater than the incident ray from normal.

Critical angle: the angle of incident ray when its angle of reflection ray is 90^o. It is called critical angle.

When the angle of incident ray is greater than the critical angle. It refracted ray goes back to the denser medium, this is called total internal reflection

Principal of Total internal refraction:

1. Light should be incident from denser to rarer medium &

2. The angle of incidence should be greater than the critical angle (c).

Usage of Total internal refraction.

-shining of diamond= the critical angle of diamond is 24.4 degree. The diamond is cut suitably such that light entering the diamond from any face falls at an angle greater than 24.4 °, suffers multiple total internal reflections at the various faces, & remains within the diamond. The diamond sparkles due to the light trapped in it.

Mirage = It is an optical illusion seen usually in deserts by which an inverted image of a distant object like a tree is observed along with the object itself as if reflected from the water - surface.

Cool air is denser and hot air is rarer. lower atmospheric layers are rarer due to higher temperature than those of upper layers. A ray of light coming from the tree bends away from the normal as it travels from denser upper layer to the rarer lower layer. This process continues till the angle of incidence is less than the critical angle. At a particular layer of air the angle of incidence becomes greater than the critical angle and it gives rise to the T.I.R. It gives the impression of a water pond near the tree i.e. mirage.

-Air bubbles in water or glass shine : Water or glass is denser w.r.t. air. When light is incident at an angle greater than the critical angle, on going from water/ glass to air bubble, it suffers T.I.R. Hence the air bubble shines brilliantly. This effect is used in manufacturing decorating glass objects.

-Totally Reflecting Prisms : These prisms are right angled isosceles prisms which turn the light through 90° or 180°. The critical angle for glass-air interface is 42° & in these prisms the angle of incidence is 45°. Thus, the angle of incidence becomes greater than the critical angle that causes total internal reflection. Such prisms are used in periscopes

Optical instruments

Microscope: It is used to magnify minute objects close to us

(i) Simple Microscope (Magnifying glass) : It is a biconvex lens. When an object is placed within its focus, a magnified, virtual and erect image is formed.

(ii) Compound Microscope : It consists of two convex lenses of unequal size placed co-axially. The larger lens is eye-lens to see the final image and the smaller lens is object lens in front of which the object is placed. The object lens forms a real inverted image eye-lens, acts as an object for the eye-lens which gives the final, virtual and highly magnified image.

DISPERSION OF LIGHT

When white light passes through a glass prism, we get seven colors on a white screen. This phenomenon is known as

Dispersion of light. These seven colors obtained are viz. violet, indigo, blue, green, yellow, orange and red. They can be remembered as ‘VIBGYOR’. Dispersion of light occurs due to refraction of light of different wavelengths contained by white light. Smaller the wavelength of light more is its deviation. Since wavelength increases from violet to red light, hence, the deviation decreases from violet to red light. A band of seven colours of white light obtained by dispersion is known as a spectrum.

Spectroscope (or Spectrometer) : A device used to disperse light and to observe the spectra obtained is called spectroscope

Colors

Color of any object is the same as is reflected by the object and received by our eyes. e.g. when white light falls on a red rose, it reflects only wavelength of red colour and absorbs all other colours of white light. Hence, it appears red.

Pigment: It imparts colour to the objects. A pigment absorbs most of the colours of white light and reflects one or more colours e.g. (i) A red rose appears black in green light. A red pigment in rose absorbs the green light and reflects nothing.

So red rose appears black. (ii) A yellow flower appears red in red light. This is because yellow pigment in the yellow flower cannot absorb red light and hence, reflects the red light.

Primary Colours : They cannot be obtained by mixing two or more colours. They are viz. Red, Blue and Green. When they are mixed together, the result is white i.e. Red + Blue + Green = White

Secondary Colours : When two primary colours are mixed, the new colour is a secondary colour.

1) Magenta (M) = Red + Blue

(2) Yellow (Y) = Red + Green

(3) Cyan (C) = Green + Blue

Complimentary Colours : Any two colours are said to be a pair of complimentary colours if these colours produce white, when mixed together e.g Yellow + Blue = White, Magenta + Green, Cyan + Red = White.

Primary Pigments : They are Yellow, Cyan & Magenta. When these three primary pigments are mixed in equal proportion a black pigment is obtained. Yellow pigment + Cyan pigment + Magenta pigment = Black pigment. Because, this mixture of pigments absorbs all colours of white light and reflects nothing, so it appears black

Secondary Pigments : When a pair of primary pigments are mixed together, a new pigment is obtained called secondary pigment. They are Cyan pigment + Yellow pigment= green pigment, Cyan pigment +Magenta pigment= Blue Pigment, Yellow+ Magenta pigment= Red pigment.

→ The primary pigments are same as that of secondary colours whereas the secondary pigments are same as that of primary colors

SCATTERING OF LIGHT

When light passes through a medium and falls on the particles of the medium, it gets scattered in all directions. The intensity of scattered light decreases with increase in the wavelength of light i.e., smaller the wavelength of light, more is the intensity of its scattering.

Tyndall effect: our atmosphere is a heterogeneous mixture of minute particles includes smoke, tiny water droplets, suspended particles of dust and molecules of air. tyndall’s effect is the scattering of light by small particles.It can be seen when a fine beam of sunlight enters a smoke-filled room through a small hole. Thus, scattering of light makes the particles visible. Tyndall effect can also be observed when sunlight passes through a canopy of a dense forest. Here, tiny water droplets in the mist scatter light.the color scattered is depend on particles, small particle scatter short wavelength light mainly blue and large particles scatter light of longer wavelength. The scattering of light relay on 1/𝛌^4.

Sky color blue: the less the wavelength the more the scattering, the light coming from sun when enter the atmosphere it get scatter and the blue light have short wavelength so it get scattered and sky seems blue.violet color and indigo color wavelength is less than the blue but sky does not look violet because amount of violet color is less and our eye have three type of cone, these cones detect red, blue and green colors so our eye is more sensitive to blue color than violet and indigo. Sea is also blue because of this.

The sun looks red at the sunrise and sunset: The red colour is scattered least due to its longest wavelength and hence, the red-light is able to travel maximum distance in atmosphere. At the time of the sunrise and sunset, the position of sun is lower in sky and hence, it looks orange-red.

Due to least scattering of red-color, it is used as a sign of danger-signals

Wave interference.

It is a phenomenon that occurs when two waves meet(superposition) while traveling along the same medium. Wave interference can be constructive or destructive in nature. When these light waves superpose(place on or above) crest to crest or trough to trough, the result is constructive interference whereas crest to trough superposition results in destructive interference

Colored soap bubbles and oil film on water surface are observed due to interference of white light.

Polarization

Polarization is a process to transform unpolarized light into polarized light. Polarized light is that in which the vibrations occur in a single plane.

Materials that polarize light. Most of them are artificial. Tourmaline is a natural Polaroid.

Uses of polarization.

-In Sun glasses to protect the eyes from glare

-In headlight of motor-car to reduce glare.

-In wind shields of automobiles

-In holography (3D Motion pictures)

-In old oil paintings to improve colour contrast

-In optical stress analysis.

-In calculators and watches, letters and numbers are formed by LCD through polarisation of light.

-Polarisation of scattered sunlight is used for navigation in solar compass in polar regions of earth, where magnetic compass becomes non-functional.

-In CD Players, polarized laser beam acts as needle for producing sound from compact disc, which is in encoded digital format

-Polarisation is used to study asymmetries in crystals and molecules using the phenomenon of optical activity. The ability of a crystal or molecule to rotate the plane of polarised light, either clockwise or anticlockwise is called optical activity.

-For 3-D view, special type of glasses, which have polaroids with perpendicular axes, are used.

Diffraction

Diffraction is the process by which a beam of light wave or other system wave spread out while passing through a narrow opening or across an edge.

sound diffraction is more pronounced than light diffraction

Coronas surrounding the sun or moon caused by diffraction

Silver lining of cloud is also caused by diffraction

The multicolor of clode is due to diffraction and known as cloud iridescence.