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Natural Phenomena due to Sunlight & Optical Instruments | Physics Class 12 - NEET PDF Download

What is Scattering of Light?


  • Light can be examined entirely from its source. When light passes from one medium to any other medium say air, a glass of water then a part of the light is absorbed by particles of the medium preceded by its subsequent radiation in a particular direction. This phenomenon is termed as a scattering of light.
    Natural Phenomena due to Sunlight & Optical Instruments | Physics Class 12 - NEET
  • The intensity of scattered light depends on the size of the particles and the wavelength of the light.
  • Shorter wavelength and high frequency scatter more due to the waviness of the line and its intersection with a particle. The wavier the line, the more are the chances of it intersecting with a particle. 
  • On the other hand, longer wavelengths have low frequency, and they are straighter and chances of colliding with the particle are less so the chances are less.
  • The bending of multicoloured light can be seen in the afternoon due to the refraction and total internal reflection of light. The wavelength of the sunlight forms different colours in different directions. 
  • Rayleigh scattering theory is reasoned for the red colour of the sun in the morning and blue colour of the sky. Let p be considered as the probability of scattering and λ is the wavelength of radiation, then it is given as:
    P ∝ 1λ4
  • The probability for scattering will give a high rise for shorter wavelength and it is inversely proportional to the fourth power of the wavelength of radiation.

Examples of Scattering of Light

  • Molecules with a larger size than the wavelength of light, experience the scattering effect differently, the phenomenon is known as Mie effect. 
  • Due to the largeness of particles, the light appears white. That is why the clouds, which are made of droplets of water are white. The blue colour is present in the major percentage among the lower wavelengths. 
  • With the wavelength of the light, the scattering efficiency of the small molecules in the atmosphere decreases. Sun radiates its light and its rays fall into the earth’s envelope thus, sunlight gets scattered in the atmosphere.
  • There are some examples that also show scattering, particles like dust, and smoke can also scatter radiation. In the same manner, we can explain the red colour appearance of the sun. 
  • For red light, the wavelength is more, and it is easy to go through the atmosphere as the scattering is less for the red light. When the light is on any other object, it gets scattered depending on its properties as different light has different intensity and each particle has different characteristics.

The Rainbow

Everyone must have seen the rainbow. The rainbow that appears in the sky is the most beautiful optical phenomenon. The sunlight passing through the water droplets present in the atmosphere undergo reflection and refraction to form a rainbow.  Sometimes after the rains two rainbows are seen. The two rainbows are the primary rainbow and secondary rainbow.


If the sunlight undergoes one internal reflection in the raindrops before emerging than the rainbow formed is the primary rainbow. Primary rainbow is brighter and narrow. It has red color at the outer edge and violet at the inner edge. The red light has a longer wavelength and is bent at least.  The violet with a shorter wavelength is bent most.

Natural Phenomena due to Sunlight & Optical Instruments | Physics Class 12 - NEET


While forming a secondary rainbow, light rays undergo two internal reflections inside the water drops and due to this, it appears to be faint. The colors are reversed in the secondary rainbow with red at the inner edge and violet at the outer edge.


Optical Instruments

An optical instrument can be defined as a device that either process light waves to enhance an image for viewing or analyzes light waves to determine one of a number of characteristic properties.
Optical InstrumentsOptical Instruments

➢ Camera

A photograph camera consists of a light proof box, at one end of which a converging lens system is fitted. A light-sensitive film is fixed at the other end of the box, opposite to the lens system. A real inverted image of the object is formed on the film by the lens system.

f-Number for a Camera: The f-number represent the size of the aperture.

f-number =Focal length of the lens (F) / Diameter of the lens(d)

Generally 2, 2.8, 4, 5.6, 8, 11, 22, 32 are f-numbers.

The amount of light (L) entering the camera is directly proportional to the area (A) of the aperture, i.e.,

L ∝A∝ d2

Brightness of Image ∝ (d2/f2)

where, d = diameter of the lens and F = focal length of the lens.

Exposure time is the time for which light is incident of photographic film.

➢ Simple Microscope

It is used for observing magnified images of objects. It is consists of a converging lens of small focal length.

Natural Phenomena due to Sunlight & Optical Instruments | Physics Class 12 - NEET

Magnifying Power

(i) When final image is formed at least distance of distinct vision (D), then M=1+d/f

where, f= focal length of the lens.

(ii) When final image is formed at infinity, then M = D/f

➢ Compound Microscope

It is a combination of two convex lenses called objective lens and eye piece separated by a distance. Both lenses are of small focal lengths but fo < fe, where fo and fe are focal lengths of objective lens and eye piece respectively

Natural Phenomena due to Sunlight & Optical Instruments | Physics Class 12 - NEET

Magnifying Power

M = vo / uo {1 + (D/fo)

Where vo= distance of image, formed by objective lens and
uo = distance of object from the objective

(ii) When final image is formed at infinity, then
M = vo/uo . D/fe

➢ Astronomical Telescope

It is also a combination of two lenses, called objective lens and eye piece, separated by a distance. It is used for observing distinct images of heavenly bodies like stars, planets etc.

Natural Phenomena due to Sunlight & Optical Instruments | Physics Class 12 - NEET

Magnifying Power

(i) When final image is formed at least distance of distinct vision (D), then M = fo/fe {1+ (D/fe)} where fo and fe are focal lengths of objective and eyepiece respectively.

Length of the telescope (L) = (fo + ue)

where, ue = distance of object from the eyepiece.

(ii) When final image is formed at infinity, then M = fo/fe

Length of the telescope (L) = fo + fe

For large magnifying power of a telescope fo should be large and fe should be small.

For large magnifying power of a microscope; fo < fe should be small.

Resolving Power

The ability of an optical instrument to produce separate and clear images of two near by objects, is called its resolving power.

Limit of Resolution

The minimum distance between two near by objects which can be just resolved by the instrument, is called its limit of resolution (d).

Resolving power of a microscope = 1/d = 2 μ sin θ / λ

where, d = limit of resolution, λ = wavelength of light used.
μ = refractive index of the medium between the objects and objective lens and θ = half of the cone angle.

Resolving power of a telescope = 1/dθ = d/1.22 λ

where, dθ = limit of resolution, A = wavelength of light used and
d = diameter of aperture of objective

Aberration of Lenses

The image formed by the lens suffer from following two main drawbacks:-

(i) Spherical Aberration: Aberration of the lens due to which the rays passes through the lens are not focussed at a single and the image of a point object placed on the axis is blurred. called spherical aberration.

It can be reduced by using

  • lens of large focal lengths
  • plano-convex lenses
  • crossed lenses
  • combining convex and concave lens

(ii) Chromatic Aberration: The image of a white object formed by lens is usually coloured and blurred. This defect of the image produced by lens is called chromatic aberration.

➢ Human Eye

Human eye is an optical instrument which forms real image of the objects on retina.

Retina colours contains lakhs of cone and rod cells which of light and intensities of light respectively.

Natural Phenomena due to Sunlight & Optical Instruments | Physics Class 12 - NEET

Ciliary muscles change the focal length of eye lens. This power of eye is called power of accommodation of eye.

Different defects of vision of human eye are described below:

(i) Myopia or Short-Sightedness It is a defect of eye due to which a person can see near by objects clearly but cannot see far away objects clearly.

In this defect, the far point of eye shifts from infinity to a nearer distance.

This defect can be removed by using a concave lens of appropriate power.

Natural Phenomena due to Sunlight & Optical Instruments | Physics Class 12 - NEET
Natural Phenomena due to Sunlight & Optical Instruments | Physics Class 12 - NEET
Natural Phenomena due to Sunlight & Optical Instruments | Physics Class 12 - NEET

(ii) Hypermetropia or Long-Sightedness In this defect, a person can see far away objects clearly but cannot see near by objects clearly.

In this defect the near point of eye shifts away from the eye.

This defect can be removed by using a convex lens of appropriate power.

Natural Phenomena due to Sunlight & Optical Instruments | Physics Class 12 - NEET
Natural Phenomena due to Sunlight & Optical Instruments | Physics Class 12 - NEET
Natural Phenomena due to Sunlight & Optical Instruments | Physics Class 12 - NEET

(iii) Astigmatism- In this defect, a person cannot focus on horizontal and vertical lines at the same distance at the same time.

Natural Phenomena due to Sunlight & Optical Instruments | Physics Class 12 - NEET

This defect can be removed by using suitable cylindrical lenses.

(iv) Colour Blindness In this defect, distinguish between few colours. a person is unable to The reason of this defect is the absence few colours. of cone cells sensitive for

This defect cannot be removed.

(v) Cataract In this defect. an opaque white membrane is developed on cornea due to which person lost power of vision partially on completely.

This defect can be removed by removing this membrane through surgery.

The document Natural Phenomena due to Sunlight & Optical Instruments | Physics Class 12 - NEET is a part of the NEET Course Physics Class 12.
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FAQs on Natural Phenomena due to Sunlight & Optical Instruments - Physics Class 12 - NEET

1. What is the scattering of light?
Ans. Scattering of light refers to the phenomenon where light rays deviate from their original path due to interaction with particles or molecules in the medium through which it passes. This can result in the scattering of light in various directions.
2. How does scattering of light cause the formation of a rainbow?
Ans. When sunlight passes through water droplets in the atmosphere, it undergoes scattering. This scattering causes the different colors of light to separate and form a circular arc of colors, known as a rainbow.
3. What are some examples of natural phenomena caused by the scattering of light?
Ans. Some examples of natural phenomena caused by the scattering of light include the blue color of the sky, the red appearance of the sun during sunrise or sunset, and the white appearance of clouds.
4. How do optical instruments utilize the scattering of light?
Ans. Optical instruments such as microscopes and telescopes utilize the scattering of light to enhance visibility and magnification. By controlling the scattering of light, these instruments can provide clearer and more detailed images.
5. How does the scattering of light affect the visibility of objects in foggy conditions?
Ans. In foggy conditions, the scattering of light by water droplets in the air reduces visibility. This is because the scattered light rays from the fog particles cause a diffused and hazy appearance, making it difficult to see objects clearly.
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