Dispersion Of Light And Natural Optical Phenomena Class 10th Science Gujarat Board Solution

Class 10^th Science Gujarat Board Solution

Exercise

1. by which optical phenomenon, the splitting of white light into seven constituent colours…
2. by which optical phenomenon, the splitting of white light into seven constituent colours…
3. Which colour of light deviates maximum in the dispersion of white light by prism?A. Violet…
4. Which of the following are the primary colours?A. Red, Blue, Yellow B. Red, Green, Violet…
5. Which of the following are primary pigments?A. Yellow, Green and Magenta B. Magenta,…
6. In human eye, the image of an object is formed at ----------------A. Iris B. Pupil C.…
7. Which is the complementary colour of blue colour?A. Red B. Yellow C. Green D. Magenta…
8. Which colour is obtained by mixing the blue and red colours?A. Green B. Magenta C. Cyan D.…
9. The focal length of an eye lens is changed due to the action of ----------------.A. Pupil…
10. Which colours are reflected when white light is incident upon blue pigment?A. Yellow,…
11. ---------- lens is used to correct the defect of vision termed as presbyopia.A. Convex B.…
12. Which phenomenon does not play a role in the formation of rainbow?A. Reflection B.…
13. For which of the following cases, the total internal reflection of light will be…
14. Where is the image in eye formed for a person suffering from defect of near-sightedness?A.…
15. Which phenomenon is responsible for the twinkling of stars?A. Atmospheric reflection B.…
16. Due to which phenomenon of light does Tyndall effect result?A. Reflection B. Refraction C.…
17. What is the time difference between actual sunset and apparent sunset?A. 2 s B. 20 s C. 2…
18. Which colour of light scatters maximum due to atmosphere?A. Blue B. Yellow C. Green D. Red…
19. Which coloured light has minimum velocity in the glass prism?A. Red B. Green C. Blue D.…
20. What is the dispersion of light? Which are the colours of the spectrum obtained from the…
21. Write the name of primary colours of light. Write the name of colours obtained from their…
22. What are called pigments? Give the name of primary pigments.
23. Write the function of ciliary muscles and retina in human eye.
24. What is an accommodation power of an eye? What is the least distance of distinct vision?…
25. What is called atmospheric refraction? Which phenomenon results from it?…
26. Give the reason for two-minute early sunrise.
27. What is the scattering of light? On what factors does it depend?
28. Why are the danger signal lights red in colours?
29. Why does the sun appear reddish at sunrise and sunset?
30. What is the total internal reflection of light? Give its illustrations.…
31. What is looming? How is it formed?
32. Explain the superposition of primary colours of light with necessary illustration.…
33. Explain the function of main parts of an eye by drawing a simple sketch of it.…
34. Explain the formation of rainbow with a neat figure.
35. Write a note on “twinkling of stars.”
36. Describe Tyndall effect.
37. Why does the clear sky appear blue in colour?
38. Explain the dispersion of white light by a glass prism using necessary figure.…
39. Describe the formation of mirage through an appropriate figure.
40. What is the defect of vision in the human eye? State its types and explain in detail.…

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Exercise

Question 1.

by which optical phenomenon, the splitting of white light into seven constituent colours occur?
A. Refraction

B. Reflection

C. Dispersion

D. Interference.

Answer:

Dispersion is the natural phenomenon by which white light is split into seven constituent colours.

Fig. Dispersion of light by prism

Question 2.

by which optical phenomenon, the splitting of white light into seven constituent colours occur?
A. Refraction

B. Reflection

C. Dispersion

D. Interference.

Answer:

Dispersion is the natural phenomenon by which white light is split into seven constituent colours.

Fig. Dispersion of light by prism

Question 3.

Which colour of light deviates maximum in the dispersion of white light by prism?
A. Violet

B. Blue

C. Green

D. Red

Answer:

In a transparent medium (like prism), the velocity of violet light is minimum which leads to maximum deviation.

Question 4.

Which of the following are the primary colours?
A. Red, Blue, Yellow

B. Red, Green, Violet

C. Yellow, Green, Blue

D. Red. Green, Blue

Answer:

Red. Green, Blue are the primary colours because every other colour can be obtained by recombining these colours.

Question 5.

Which of the following are primary pigments?
A. Yellow, Green and Magenta

B. Magenta, Yellow and Cyan

C. Blue, Green and Violet

D. Red, Green and Yellow

Answer:

Magenta, Yellow and Cyan are the primary pigments.

Question 6.

In human eye, the image of an object is formed at ----------------
A. Iris

B. Pupil

C. Retina

D. Cornea

Answer:

The light rays after being refracted from the eye lens falls on the retina where the image is formed.

Question 7.

Which is the complementary colour of blue colour?
A. Red

B. Yellow

C. Green

D. Magenta

Answer:

Magenta is the complementary colour of blue.

Question 8.

Which colour is obtained by mixing the blue and red colours?
A. Green

B. Magenta

C. Cyan

D. Yellow

Answer:

Magenta colour is obtained by mixing the blue and red colours.

Question 9.

The focal length of an eye lens is changed due to the action of ----------------.
A. Pupil

B. Retina

C. Cilliary muscles

D. Cornea

Answer:

The focal length of an eye lens is changed due to the action of ciliary muscles.

Fig. Function of ciliary muscles

Question 10.

Which colours are reflected when white light is incident upon blue pigment?
A. Yellow, Orange, Green

B. Violet, Green, Blue

C. Violet, Yellow, Green

D. Yellow, Green, Blue

Answer:

When the white light is incident on the blue pigment Violet, Green, Blue are reflected while the other colours are absorbed. This is known as colour subtraction.

Question 11.

---------- lens is used to correct the defect of vision termed as presbyopia.
A. Convex

B. Concave

C. Bifocal

D. Contact

Answer:

Presbyopia is the decrease in accommodation power with ageing which can be corrected by bifocal lenses.

Question 12.

Which phenomenon does not play a role in the formation of rainbow?
A. Reflection

B. Refraction

C. Dispersion

D. Absorption

Answer:

Absorption does not play a role in the formation of rainbow.

Question 13.

For which of the following cases, the total internal reflection of light will be possible?
A. Angle of incidence is less than critical angle.

B. Angle of incidence is equal to critical angle.

C. Angle of incidence is more than critical angle.

D. Angle of incidence equal to angle of refraction.

Answer:

Total internal reflection occurs when light rays travels from a medium of greater refractive index to a medium of lower refractive index and the angle of incidence must be greater than the critical angle.

Fig. Total internal reflection

Question 14.

Where is the image in eye formed for a person suffering from defect of near-sightedness?
A. On retina

B. On backward region of retina

C. On a region ahead of retina

D. On pupil.

Answer:

For a person suffering from defect of near sightedness the light rays meet ahead of the retina which is corrected by using a concave lens.

Fig. Image correction in myopia

Question 15.

Which phenomenon is responsible for the twinkling of stars?
A. Atmospheric reflection

B. Atmospheric refraction

C. Reflection

D. Total internal reflection

Answer:

The phenomenon responsible for the twinkling of the stars is atmospheric refraction which occurs due to the dense air in the earth’s atmosphere. Due to mobility and the change in temperature of the air the refractive index keeps on changing continuously and randomly. So, the apparent position of the star is not steady and it appears to be twinkling.

Question 16.

Due to which phenomenon of light does Tyndall effect result?
A. Reflection

B. Refraction

C. Scattering

D. Dispersion

Answer:

Scattering is the natural phenomenon where the light rays are directed in different directions when it interacts with small particles.

Question 17.

What is the time difference between actual sunset and apparent sunset?
A. 2 s

B. 20 s

C. 2 minute

D. 20 minute

Answer:

The time difference between actual sunset and apparent sunset is 2 minutes.

Question 18.

Which colour of light scatters maximum due to atmosphere?
A. Blue

B. Yellow

C. Green

D. Red

Answer:

The particles present in the atmosphere have the same size as the wavelength of blue light, so the blue light is scattered more by the fine atmospheric particles.

Question 19.

Which coloured light has minimum velocity in the glass prism?
A. Red

B. Green

C. Blue

D. Violet

Answer:

Red light has the longest wavelength so it travels the fastest.

Question 20.

What is the dispersion of light? Which are the colours of the spectrum obtained from the dispersion through a glass prism?

Answer:

Dispersion is the natural phenomenon by which white light is split into seven constituent colours. The band of the seven colours obtained from the splitting of white light is called spectrum. All the constituents of white light travelling in vacuum they have the same velocity but when they pass through a transparent medium their velocity changes and they tend to deviate.

Fig. Dispersion of white light by a prism

The colour spectrum obtained from the dispersion of white light are:

Ø V – Violet

Ø I – Indigo

Ø B – Blue

Ø G – Green

Ø Y – Yellow

Ø O – Orange

Ø R - Red

Question 21.

Write the name of primary colours of light. Write the name of colours obtained from their mixture?

Answer:

The primary colours are Red, Green and Blue.

The colours obtained from their mixture are given by the table:

Question 22.

What are called pigments? Give the name of primary pigments.

Answer:

The coloured substances that are used as paint are known as pigments. The paints used for painting infrastructures and drawing are well-known examples of pigments.

The primary pigments are cyan, magenta and yellow.

Question 23.

Write the function of ciliary muscles and retina in human eye.

Answer:

The muscular structures which hold the eye lens in its position are known as ciliary muscles, they adjust the focal length of the lens by adjusting its thickness. The position where the image is formed due to refraction from the eye lens is called retina, the light-sensitive cells in the retina generate electric signals which are then transferred to the lens to interpret.

Question 24.

What is an accommodation power of an eye? What is the least distance of distinct vision?

Answer:

The ability of the eye lens to adjust its focal length as per requirement is called accommodation power of lens.

The ciliary muscles modify the curvature of the lens which in turn modifies the focal length of the eye lens. In the relaxed state of the ciliary muscles, the lens is thin and the focal length is more which enables us to see distant objects clearly. When the ciliary muscles contracts, the focal length decreases due to increased curvature of the lens and we are able to see near objects clearly.

The least distance of distinct vision for a young adult with normal vision is 25cm.

Question 25.

What is called atmospheric refraction? Which phenomenon results from it?

Answer:

The density of earth’s atmosphere is not same everywhere, the layer near the earth’s surface has more density than the air at higher altitude. So, the refractive index keeps on changing continuously and randomly. Hence, the apparent position of the objects in distant outer space also keeps on changing. This phenomenon is known as atmospheric refraction.

The phenomenon such as twinkling of stars, early sunrise and delayed sunset occurs due to this effect.

Question 26.

Give the reason for two-minute early sunrise.

Answer:

Actual sunrise occurs when sun appears at the horizon.

During sunrise even the sun is lower than the horizon, the sun rays due to atmospheric refraction reaches our eyes and we experience sunrise before two minutes.

Fig. Apparent position of sun during sunrise.

Question 27.

What is the scattering of light? On what factors does it depend?

Answer:

Scattering is the natural phenomenon where the light rays are directed in different directions when it interacts with small particles.

Scattering depends on the size of the particles, if the particles are very small then it will scatter light of smaller wavelength (blue light) and if the particle size is slightly bigger then it will scatter light of greater wavelength (red light). If the particles are much bigger in size then they will scatter white light.

Question 28.

Why are the danger signal lights red in colours?

Answer:

The red colour light is scattered least by the smoke or fog, so it can be seen from a long distance. Hence, the red light is used in danger signals.

Question 29.

Why does the sun appear reddish at sunrise and sunset?

Answer:

During sunrise and sunset the sun rays (white light) have to travel a greater distance compared to any time of the day. While travelling the blue light of the sunrays gets scattered, only the red light reaches our eyes. Hence, at sunrise and sunset the sun appears red.

Question 30.

What is the total internal reflection of light? Give its illustrations.

Answer:

When the light rays travel from an optically denser medium to an optically rarer medium, the light rays are completely reflected back to the denser medium if the angle of incidence is greater than the critical angle. This phenomenon is known as total internal reflection.

Sparkling of diamond and working of optical fibre are both governed by this principle.

Fig. Total internal reflection in an optical fibre

Question 31.

What is looming? How is it formed?

Answer:

Looming is a kind of mirage observed in very cold region in which the distant objects appear to be hanging midway in the air, virtual and erect. This is caused due to total internal reflection in downwards direction caused by atmospheric refraction when the warmer (optically rarer) air remains above the colder (optically denser) air in the atmosphere.

Fig. Looming

Question 32.

Explain the superposition of primary colours of light with necessary illustration.

Answer:

The primary colours red, blue and green can be combined in various combinations and proportions to get different colours. This phenomenon is known as superposition of primary colours.

The method of producing a wide range of colours by the appropriate mixture of this primary colours is known as additive mixture method.

Fig. Mixing of primary colours of light

Question 33.

Explain the function of main parts of an eye by drawing a simple sketch of it.

Answer:

The function of the main parts of an eye is:

1. Iris: The muscular diaphragm behind the cornea that controls the amount of light that enters the eye.

2. Pupil: The aperture of an eye behind the cornea whose size can be controlled by the iris.

3. Ciliary muscles: The muscular structures that hold the eye lens in its position and also control their aperture.

4. Retina: It is a light-sensitive tissue where the image is formed after refraction.

Fig. Structure of eye

Question 34.

Explain the formation of rainbow with a neat figure.

Answer:

The rainbow in a natural spectrum visible in the sky after rainshower. It is formed by the dispersion of light by the tiny droplets present in the atmosphere, the water droplets act like prism. It disperses the incident light first, and then reflects it internally (not necessarily total internal reflection) and finally refract it again while it comes out of raindrop.

Fig. Formation of rainbow

Question 35.

Write a note on “twinkling of stars.”

Answer:

The phenomenon responsible for the twinkling of the stars is atmospheric refraction which occurs due to the dense air in the earth’s atmosphere. Due to mobility and the change in temperature of the air the refractive index keeps on changing continuously and randomly. So, the apparent position of the star is not steady and it appears to be twinkling.

Question 36.

Describe Tyndall effect.

Answer:

Tyndall effect is the phenomenon in which the path of the light ray is visible when it travels in a colloidal solution.

Examples: light rays visible in dense forest or a smoke-filled room.

Question 37.

Why does the clear sky appear blue in colour?

Answer:

When the sun rays travel through the atmosphere it interacts with small particles in the air that leads to scattering of the sun rays and blue light is scattered. This makes the sky to appear blue in colour blue.

Question 38.

Explain the dispersion of white light by a glass prism using necessary figure.

Answer:

Dispersion is the natural phenomenon by which white light is split into seven constituent colours. The band of the seven colours obtained from the splitting of white light is called spectrum. All the constituents of white light travels in vacuum they have the same velocity but when they pass through a transparent medium their velocity changes and they tend to deviate.

Fig. Dispersion of white light by a prism

The colour spectrum obtained from the dispersion of white light are:

Ø V – Violet

Ø I – Indigo

Ø B – Blue

Ø G – Green

Ø Y – Yellow

Ø O – Orange

Ø R - Red

Question 39.

Describe the formation of mirage through an appropriate figure.

Answer:

Mirage is a type of optical illusion that is generally seen in deserts and in very cold regions.

When the light rays coming from tall objects pass through the air of gradually decreasing refractive index( as the air gradually becomes hot), the light rays bend gradually away from the normal and their angle of refraction increases gradually so that they enter observer’s eye after total internal reflection occurs. Hence, the image formed in a mirage is virtual and upside down.

Fig. Diagram of a mirage

Question 40.

What is the defect of vision in the human eye? State its types and explain in detail.

Answer:

Defects of human eye is simply the inability of the ciliary muscles to change the thickness of the eye lens. They can be classified into three types:

1. Near-sightedness: Myopia or near sightedness is an eye defect in which a person can see clearly only when the objects are near. This happens when the lens becomes too thick and the light rays undergo greater refraction which makes the light rays to converge before the retina, making the eyesight blurry.

Using a concave lens can be helpful as it concentrates the light rays on the retina. The below image explains the process

Fig. Correction of myopia

2. Hypermetropia: Hypermetropia or far-sightedness is an eye defect there is difficulty with near vision but far objects can be seen easily. The image is focused behind the retina rather than upon it. This occurs when the eyeball is too short or the refractive power of the lens is too weak. Hypermetropia can be corrected by wearing glasses/contacts that contain convex lenses.

Fig. Correction of Hypermetropia

3. Presbyopia: Presbyopia is the decrease in accommodation power with ageing which can be corrected by bifocal lenses. This defect is caused due to weakening of ciliary muscles and loss of elasticity of the eye lens. This can be treated by using bifocal lens.