Question 25: Explain the structure and functioning of Human eye. How are we able to see nearby as well as distant objects?
Answer: The human eye is a spherical structure which fits in the eye socket in the skull bone. There are following main parts in the human eye.
Pupil: Pupil is the round black spot in front of eye. It regulates the amount of light entering the eyes.
Iris: Iris is made of muscles. These muscles control the size of opening of pupil.
Lens: Lens lies just behind the pupil. Lens focuses on objects at different distances so that clear image can be formed on retina.
Retina: Retina works like a screen or camera film. Retina is full of light and colour sensitive cells. These cells, upon receiving image send electrical signals to the brain, which processes these information to make a mental image of what we see.
Power of Accommodation of Human Eye: The human eye can clearly see a nearby object as well as an object on infinity. This ability of the human eye is called the power of accommodation of human eye.
Question 26: When do we consider a person to be myopic or hypermetropic? Explain using diagrams how the defects associated with myopic and hypermetropic eye can be corrected?
Answer: Myopia: Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself.
Correction of Myopia: This defect can be corrected by using a concave lens of suitable power. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
Hypermetropia: Hypermetropia is also known as far-sightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. This happnens because the light rays from a nearby object are focused at a point behind the retina.
Correction of Hypermetropia: This defect can be corrected by using a convex lens of appropriate power. Eye-glasses with converging lenses provide the additional focusing power required for forming the image on the retina.
Question 27: Explain the refraction of light through a triangular glass prism using a labeled diagram. Hence define the angle of deviation.
Answer: When a ray of light enters a prism, it bends towards the normal because it is entering from a rarer medium to a denser medium. After that, when this ray of light comes out of prism to enter air, it bends away from normal.
Due to triangular shape of the prism, the emerging ray makes and angle with the incident ray. This angle is called the angle of deviation.
Question 28: How can we explain the reddish appearance of sun at sunrise or sunset? Why does it not appear red at noon?
Answer: (Note: This question is similar to question 24)
Question 29: Explain the phenomenon of dispersion of white light through a glass prism, using suitable ray diagram.
Answer: In the answer to question 27, you have read about refraction of light through prism. Due to the angle of the prism and due to different wavelengths of different components of white light, the emergent ray gets segregated into different colours of visible spectrum. Finally, a colourful band of seven colours is obtained. This phenomenon is called dispersion of white light by the prism.
Question 30: How does refraction take place in the atmosphere? Why do stars twinkle but not the planet?
Answer: When light enters from one medium to another, there is a deviation in its path. This phenomenon is called refraction of light. Atmosphere is composed of layers of various optical densities. Because of this, light rays (passing through various layers of atmosphere) get deviated.
From the earth, stars are at huge distance compared to planets. So, a star acts like a point-sized source of light. But planets do not act like point-sized sources of light because of their proximity to us. Due to this, stars appear to twinkle but planets do not twinkle.
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