(a) A student is unable to see clearly the words written on the black board placed at a distance of approximately 3 m from him. Name the defect of vision the boy is suffering from. State the possible causes of this defect and explain the method of correcting it.

(b) Why do stars twinkle? Explain.

OR

(a) Write the function of each of the following parts of human eye:

(i) Cornea (ii) Iris (iii) Crystalline lens (iv) Ciliary muscles

(b) Why does the sun appear reddish early in the morning? Will this phenomenon be observed by an astronaut on the Moon? Give reason to justify your answer.

(a) As the student is unable to clearly see distant objects, it suggests that the student is suffering from an eye defect. The defect is referred to as Myopia or short sightedness. The far point for a myopic person is closer than the infinity [fig(a)]. This defect is caused by elongation of the eye ball or due to excessive curvature of the eye lens. The image formed in this case is in front of retina but not at the retina itself resulting into the blurriness of the image.

This defect is corrected using a concave lens that diverges the ray coming from the object and shift the image to the retina, which in turn improves the clarity of the image produced by the eye. As shown in the figure the concave lens diverges the incoming rays thereby fixing the aberration.

(b) The twinkling of the star is governed by the phenomena of refraction due to the earth’s atmosphere. As the light originating from the star enters the earth’s atmosphere, it is refracted by a certain angle, which bends the light. The bending of the light causes the star to change its position and the brightness, hence the twinkle.

OR

(a)

Fig – Human Eye

(i) Cornea: Cornea is a smooth, thin dome shaped membrane that covers the pupil, iris and anterior chamber a human eye. The primary function of the cornea is to let the incoming ray enter the eye and converge the light rays. It accounts for almost one-third of the power of the eye lens. Also, cornea protects the eye from minor injury and abrasions.

(ii)Iris: The iris is a pigmented tissue located behind the cornea that gives colour to the eye. It is responsible for regulating the amount of light entering the eye by varying the size of the pupil and also perpetually adapting to the light changes in the surrounding. The iris allows the near point reading.

(iii)Crystalline lens: Human eye consists of clear jelly-like transparent material, shaped as a convex lens that converges the light and brings it to the retina for vision. It is suspended by the ligaments attached in between the ciliary body that varies the curvature of the lens, which in turn changes the power.

(iv)Ciliary muscles: Ciliary muscles are composed of tissue located at the periphery of the eye. The role of ciliary muscles is to change the curvature of the lens that can thus change the focal length. When you are looking at an object placed closer to the eye, the ciliary muscles contract. The contraction of muscles increases the curvature of the lens as a result the lens become thicker; as a result the focal length of the eye lens decreases. Thus, it enable us to see the object clearly.

(b) The sun appears reddish early in the morning due to the refraction of the light rays. At the time of sun rise the light rays travel a longer distance before reaching our eye. As a result the light rays with shorter wavelength gets scattered away by the thick layer of atmosphere at the horizon, leaving only the rays of longer wavelength i.e. Red. During the day time, the sun is situated overhead. Therefore, the only a small fraction of blue and violet colour of shorter wavelength get dispersed in the atmosphere and the sun appears to be yellow.