CBSE Class 10 Physics Human Eye Colorful World Worksheet B

Question : How does the refractive index of earth’s atmosphere vary with height?
Answer : The hotter air in the atmosphere is lighter than the cooler air and has a refractive index slightly less than that of the cooler air. The refractive index of the upper hotter atmosphere is therefore less than the cooler layers of atmosphere below it.

Question : Define power of a lens. 
Answer : Power (P), of a lens is the reciprocal of its focal length. Power = 1/f , where f is the focal length of the lens.

Question : Why does the emergent ray in a prism bend at an angle to the direction of the incident ray?
Answer : The emergent ray in a prism bends at an angle to the direction of the incident ray as both the refracting surfaces in a prism are inclined to each other. 

Question : Give one main difference between the lens of human eye and lens of camera.
Answer : Lens of human eye has flexible aperture, its focal length can be changed. In camera focal length can not be changed for a lens.

Question : What is the function of crystalline lens of human eye?
Answer : The crystalline lens provides the proper focal length required to focus objects at different distances on the retina.

Question : Name the transparent membrane through which light enters first in the eye.
Answer : Cornea.

Question : What is persistence of vision?
Answer : When the image is formed on retina it remains there for 1/16th of a second and this property of eye is called persistence of vision.

Question : Define angle of prism.
Answer : The angle formed due to two lateral faces of the prism is called the angle of prism.

Question : What is the function of iris?
Answer : Iris controls the size of the pupil.

Question : What is presbyopia?
Answer : It is the defect of eye in which one cannot see nearby as well as far objects clearly.

Question : What is dispersion of light?
Answer : The splitting of light into its various components (i.e., 7 colours) is called dispersion of light.

Question : What type of signals are generated and sent to the brain by light sensitive cells of retina?
Answer : Electrical signals.

Question : Which part of the human eye controls the amount of light entering the eye?
Answer : Pupil.

Question : In visible spectrum which colour has longest wavelength.
Answer : Red.

Question : What holds the crystalline lens in the human eye?
Answer : Ciliary muscles.

Question : Why is inverted imageformed on the retina of human eye?
Answer : The inverted image is formed due to the eye lens which is convex in shape. Through it the light rays enters to form the real, inverted image.

Question : In which type of eye defect near point of the eye becomes more than 25 cm?
Answer : Hypermetropia.

Question : How does the size of particles present in medium produce different colours of light by its scattering property?
Answer : The colour of the scattering light in a medium depends on the size of the scattering particles.
(i) If the size of particles is very small, it will scatter mainly blue light.
(ii) If the size of particles is larger then it will scatter light of longer wavelength i.e., red.
(iii) If the size of scattering particles is larger enough, the light appears to be white.

Question : What is meant by dispersion of white light? Draw a ray diagram to show the dispersion of white light by a glass prism. Give reason why do we get different colours of light?
Answer : Dispersion of light: The splitting of white light into seven colours on passing through a transparent medium like glass prism is called dispersion of light.
Diagram : Dispersion of light

We get different colours because each colour of light has different bending ability when they pass through the glass prism.

Question : In presbyopia, we use bi-focal lens with upper portion concave lens and lower portion convex lens. Why is the arrangement so?
Answer : The upper portion is concave lens so that our eyes can see the distant objects. When the rays come parallel from infinity it will pass through the upper part of spectacles.
Whereas for the lower part consists of convex lens which facilitates our eye to see the near objects.

Question : Distinguish between presbyopia and hypermetropia.
Answer : 

Question : To correct myopia why we use concave lens and to correct hypermetropia, why do we use convex lens? Why can’t we do vice-versa?
Answer : Myopia is a defect in which the image is formed in front of retina. We need to use diverging lens so that it can further diverge the light rays before it enters our eye and make it possible to meet on the retina.
In case of hypermetropia, the image is formed beyond retina, we need to use convex lens so as to converge the rays and make it possible for the rays to meets on the retina.

Question : Give the difference between myopia and hypermetropia.
Answer : 

Question : Why does light splits into spectrum when it passes through prism only and does not split when it passes through glass slab?
Answer : Rectangular glass slab has parallel refracting surfaces, the emergent ray is parallel to the incident ray. It slightly gets displaced laterally. While in case of prism the surface is not parallel and the light ray gets deviated at larger angle due to which it splits into its constituent colour.

Question : What is internal reflection?
Answer : When a light rays enters from one medium to another (e.g., rarer to denser i.e., air to water droplet) then a ray of light instead of passing through it reflects in the second medium then it is said to be internal reflection of light.

Question : Give one use of the following properties of light:
(i) Scattering of light (ii)Persistence of vision
(iii) Power of accommodation (iv) Refraction of light
(v) Reflection of light
Answer : (i) Due to scattering of light we can see the different colours of sky and rainbow formation.
(ii) Used in cinematography.
(iii) Eye can see both nearby and far off objects from same point.
(iv) We can see through lenses, eye defects can be corrected, we can see under water.
(v) Our eyes can see object only due to reflection of light by the object into our eyes.

Question : Give reason for early sunrise and delayed sunset.
Answer : 

Sun being far off the light rays entering our eye gets refracted several times due to the atmosphere and the sunrise and sunset are seen to us due to the bending of light and that light enters our eye to visualise the sun at that particular point.