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Q 1. When a person is suffering from both myopia and hypermetropia, then what type of correction lenses are required?

Solution

A bi-focal lens is required for this defect. It is also known as Presbyopia. This defect is common in old age.

Q 2. When you travel by an airplane flying at high attitude, the sky appears almost dark. Why?

Solution

When we travel by an airplane flying at high attitude, the sky appears almost dark because there is no atmosphere at higher altitudes. Thus, there are no particles to scatter light at such altitude. This makes the sky look black.

Q 3. What is the color sequence in a pure spectrum?

Solution

The color sequence is: Violet, Indigo, Blue, Green, Yellow, Orange and Red i.e., VIBGYOR.

Q 4. Which colours of the spectrum is least and most scattered?

Solution

Light of shorter wavelength is easily scattered as compared to light of longer wavelength. Red light has longest wavelength, and hence it is least scattered. Violet light has the shortest wavelength, and hence it is the most scattered light.

Q 5. Time from sunrise to sunset is lengthened by how many minutes?

Solution

We see the sun rising two minutes earlier than the actual rise and see it setting two minutes after the actual sunset. This happens due to atmospheric refraction. Hence, the total time by which the time from sunrise to sunset is lengthened, is 2 + 2 = 4 minutes.

Q 6. What is the nature of the image formed on the retina of the eye?

Solution

The image is real, inverted and diminished.

Q 7. Why the sun appears red at sunrise and sunset?

Solution

At the time of sunrise and sunset, when the sun is near the horizon, the sunlight travels the greatest distance through the atmosphere to reach us. During this, most of the shorter wavelengths present in it are scattered away from our line of sight. So, the light reaching us directly from the rising sun or setting sun consists mainly of longer wavelength red color due to which the sun appears red. Thus, at sunrise and sunset the sun itself as well as the surrounding sky appears red.

Q 8. What is called atmospheric refraction? Which phenomenon results from it?

Solution

Earth’s atmosphere consists of layers of different densities and hence different refractive indices. The lower layers being denser than the upper ones. Thus, the light from objects is refracted at each layer of the atmosphere. In addition, physical situations like temperature are not steady; thus, the apparent position of the object also fluctuates. This phenomenon of refraction of light due to layers of atmosphere of varying densities is called atmospheric refraction. Phenomenons like ‘twinkling of stars’, ‘early sunrise’ and ‘delayed sunset’ occur due to atmospheric refraction.

Q 9. Define: i) Far point of eye ii) Near point of eye

Solution

Far point of the eye: The farthest point up to which the eye can see objects clearly is called the far point of the eye. Near point of eye: The minimum distance at which objects can be seen most distinctly without strain is called the least distance of distinct vision. It is also called the near point of the eye.

Q 10. Light of two colours A and B pass through a prism. A deviates more than B from its path of incidence. Which colour has a higher speed in the prism?

Solution

B deviates more than A. Thus, B has a higher speed.

Q 11. Atmospheric refraction results into some atmospheric phenomena. State some examples.

Solution

Atmospheric phenomena caused due to atmospheric refraction are: 1. Twinkling of stars 2. Advance of sunrise and delay in sunset.

Q 12. Which color of white light has higher wavelength? What is the relation between smallest and largest wavelength?

Solution

Out of seven colors of white light, the wavelength of violet color is minimum (about 400 nm) and that of red color is maximum (about 700 nm). Thus, wavelength of red light is about 1.8 times the wavelength of violet light.

Q 13. How do ciliary muscles affect the functioning of eye?

Solution

Ciliary muscles can contract or relax. Because of this, they can change the thickness of the eye lens, thereby changing its focal length. With proper change in the focal length, the eye lens can produce an image of nearby objects and also far away objects. So, the power of accommodation of the eye is due to the function of the ciliary muscles.

Q 14. What is vision? State the three common defects of vision.

Solution

The ability to see is called "Vision". The three common defects of vision are 1. Myopia 2. Hypermetropia 3. Presbyopia

Q 15. Do planets twinkle at night? Explain.

Solution

No, planets do not twinkle at night because, like stars they are not point sources of light. Instead, they are like a collection of Point sources of light, that is, they are extended sources of light. Hence, the overall effect of all the sources is a continuous brightness of the planets.

Q 16. Name the part of the eye that has real, inverted image of the object formed on it.

Solution

Retina is the part of eye where the image is formed. It acts as a screen.

Q 17. While sitting in the last row, a student has difficulty in reading the black board clearly. State the defect of vision the student is suffering from. Mention two causes of this defect. Suggest a suitable lens for the correction of this defect.

Solution

(i) Myopia or shortsightedness. (ii) Two causes of this defect are: 1. increase in the length of the eyeball 2. decrease in focal length of the eye lens (iii) Concave lens.

Q 18. What is meant by accommodation of eye? Name the part of eye which helps in this phenomenon and state how does it help.

Solution

(i) The ability of eye lens to adjust its focal length is accommodation. (ii) The part of eye that helps in this is, ciliary muscles. While looking at distant objects muscles relax and focal length increases; while looking at nearby objects muscles contract and focal length decreases.

Q 19. Name any two phenomenon associated with the formation of rainbow.

Solution

  The phenomenon associated with formation of rainbow is: (Any two) (i) Dispersion of sunlight by water droplets present in the atmosphere. (ii) Total internal reflection. (iii) Refraction of light.

Q 20. What are the causes of the following defects of vision and how can they be corrected? (a) Cataract (b) Presbyopia

Solution

(a) Cataract: Cause - sometimes the crystalline lens becomes milky and cloudy in old age. This causes partial or complete loss of eyesight. Correction - Through a cataract surgery. (b) Presbyopia : Cause - The power of accommodation of the eye decreases with age. So the near point recedes away. It arises due to weakening of ciliary muscles and diminishing flexibility of the eye lens. Sometimes a person may suffer from both myopia and hypermetropia. Correction - By wearing spectacles having bi-focal lens.

Q 21. What is total internal reflection of light?

Solution

For a ray of light travelling from denser to rarer medium, when the angle of incidence in the denser medium is greater than critical angle, the light ray gets reflected into the denser medium at the interface i.e., light does not enter the rarer medium. This phenomenon is called total internal reflection.

Q 22. Give reason: It is possible to enjoy a rainbow at fountains in any season.

Solution

Rainbow can be seen as a combined result of refraction, dispersion and reflection of sunlight by water droplets present in the atmosphere after it has rained. When there is no rain a fountain can serve as a source of fine droplets. These water droplets, in sunlight, can produce a rainbow. So it is possible to enjoy a rainbow at fountains in any season.   

Q 23. Why does the colour of the sky appear blue? Explain in brief.

Solution

The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. Light of shorter wavelength at the blue end is scattered more than light of longer wavelength at the red end. So, when sunlight passes through the atmosphere, the fine particles in air scatter the blue colour the most.

Q 24. Provide an example to show the atmospheric refraction.

Solution

We can see the atmospheric refraction during summer when the objects get heated up by the sun rays. The air closer to the object is less dense than the air farther from it. So the light travels from lighter to denser medium and object seems to be moving due to atmospheric refraction.

Q 25. (a) Mention the reasons for the following phenomena: (i) Dispersion of light (ii) Twinkling of stars (iii) Tyndall effect (iv) Blue colour of the sky. (b) How would the sky appear in the absence of earth's atmosphere?

Solution

(a) (i) Difference in speed in medium for different wavelengths/colours of light. (ii) Atmospheric refraction (iii) Scattering of light by colloidal particles (iv) Atmospheric scattering. (b) In the absence of earth's atmosphere, there will not be any scattering of light, and so the sky will appear dark.

Q 26. State 2 old age defects.

Solution

The old age defects are: (i) Presbyopia - is a old age defect in which the old person cannot see the nearby objects. (ii) Cataract - is a condition in which the person has blurred vision as lens of the eye becomes cloudy.

Q 27. A person is able to see objects clearly only when placed at distance between 50 cm and 300 cm from his eye. (a) What kind of defect of vision is he suffering from? (b) What kind of lens will be required to increase his range of vision from 25 cm to infinity? Explain briefly

Solution

(a) The person is suffering from both myopia and hypermetropia. (b) He has to use bi-focal lens to increase his range of vision from 25 cm to infinity. The upper portion (concave lens) facilitates distant vision and the lower portion (convex lens) facilitates near vision.

Q 28. Name the following parts of a human eye - (1) A thin membrane through which light enters the eye. (2) Dark muscular diaphragm that controls the size of the pupil.

Solution

(1) Cornea (2) Iris

Q 29. What is meant by near point of a human eye?

Solution

The minimum distance at which objects can be seen most distinctly without strain, is called the near point of the eye. It is 25 cm for normal vision.

Q 30. Name the part of the human eye that helps in changing the focal length of the eye lens.

Solution

Ciliary muscles help in changing the focal length of the eye lens.

Q 31. Suggest an experiment to produce rainbow in your classroom and explain procedur

Solution

Select a white coloured wall on which the sun rays fall. Stand in front of a wall such a way that the sun rays fall on your back. Hold a tube through which water is flowing. Place your finger in the tube to obstruct the flow of water. Water comes out through the small gaps between the tube and your finger like a fountain. While you are showering the water, you can see the colours of rainbow on the wall. The beautiful colours of rainbow are due to dispersion of the sunlight by millions of tiny water droplets.

Q 32. Explain: Colour blindness

Solution

It is an eye defect due to which a person is unable to distinguish between certain colours. It is an genetic disorder.

Q 33. Give three differences between myopia and hypermetropia.

Solution

Myopia Hypermetropia i. Near by objects are seen clearly but not the distant objects. i. Far away objects are seen clearly but not the nearby objects. ii. Image is formed in front of retina. ii. Image is formed behind retina. iii. Can be corrected by using diverging (concave) lens. iii. Can be corrected by using converging (convex) lens.

Q 34. How do we see moving pictures on a cinema screen?

Solution

The image of an object lasts on retina for 1/16 th second. In cinema, pictures move with speed of 24 pictures or frame per second. So the next picture is seen before the image of first has died and the pictures appear as moving on the screen. This is due to persistence of vision.

Q 35. Name the component of white light that deviates (i) the least and (ii) the most while passing through a glass prism.

Solution

Red deviates the least, Violet deviates the most.

Q 36. Stars twinkle while the planets do not twinkle. Why?

Solution

Continuously changing physical conditions of atmosphere refracts light from the stars, which are considered as point sources, by different amounts from one moment to the other - when atmosphere refracts more starlight towards us the star appears to be bright and when the atmosphere refracts less starlight then the stars appears to be dim. However the planets are nearer to us than the stars, and hence they can not be considered as point sources, hence no twinkling.

Q 37. What is the color of scattered sunlight when the size of the scattering particles is 
relatively large?
Solution

If the size of the scattering particles is relatively large enough,then,the scattered light will appear white.

Q 38. What do you mean by blind spot?

Solution

The blind spot is a small area of the retina insensitive to light where the optic nerve leaves the eye.

Q 39. Why are the traffic light stop signals are of red color?

Solution

In the visible spectrum, the red color has the largest wavelength. The red colour is least scattered by fog or dust particles. So we can observe red colour easily even in foggy conditions. That is why stop signals have red colour.

Q 40. Explain how the power of accommodation of human eye is achieved.

Solution

The eye lens of human eye is made up of a crystalline lens of variable focal length. The focal length of human eye can be changed by the action of ciliary muscles to form the images of the objects of different positions on the retina by relaxing the ciliary muscles for distant objects and by contracting the ciliary muscles for nearby objects.