CBSE Class 10 Science Light and the Human Eye Worksheet Set 04

Answer: (a) virtual. A rear-view mirror is convex and always forms a virtual, erect and diminished image.

Teacher's Note:
1. The convex mirror used as a rear-view mirror gives a wide field of view.
2. Its image is always virtual, erect and smaller than the object.

Answer: (d) Red. Red has the longest wavelength in the visible spectrum, so it slows down least in glass and bends least.

Teacher's Note:
1. Speed of light in glass is greatest for red and least for violet.
2. That is why violet bends the most and red the least.

Answer: The power of a lens is the reciprocal of its focal length expressed in metres: P = 1/f. Its SI unit is the dioptre (D); 1 D is the power of a lens of focal length 1 m.

Teacher's Note:
1. A convex lens has positive power and a concave lens has negative power.
2. Always convert focal length into metres before using P = 1/f.

Answer: The pupil is the opening in the centre of the iris. It controls the amount of light entering the eye: it becomes smaller in bright light and larger in dim light.

Teacher's Note:
1. The iris muscles change the size of the pupil.
2. This protects the retina from too much light.

Answer: When light falls on very small particles such as air molecules, dust or water droplets, the particles absorb part of the light and re-emit it in different directions. This spreading of light in all directions is scattering. Short wavelengths (blue) scatter much more than long wavelengths (red).

Teacher's Note:
1. Scattering explains the blue colour of the sky and the red colour of the Sun at sunrise and sunset.
2. The amount of scattering is greater for shorter wavelengths.

Answer: The colour of scattered light depends on (i) the size of the scattering particles and (ii) the wavelength of the incident light. Very fine particles scatter mainly blue light, while larger particles scatter longer wavelengths, so that the scattered light looks whitish.

Teacher's Note:
1. Fine particles in the atmosphere make the sky blue.
2. Large particles such as water droplets in clouds scatter all colours and make clouds look white.

Answer: (i) Object beyond C: the image is formed between F and C; it is real, inverted and smaller than the object.
(ii) Object between P and F: the image is formed behind the mirror; it is virtual, erect and magnified.

Teacher's Note:
1. Draw ray diagrams using the ray parallel to the axis and the ray through the centre of curvature.
2. In case (ii) the reflected rays diverge, so the image is seen only when they are extended backwards.

Answer: For a given pair of media and a light of a particular colour, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant: sin i / sin r = constant. This constant is the refractive index of the second medium with respect to the first.

Teacher's Note:
1. Snell's law is also called the second law of refraction.
2. The constant depends on the media and on the colour of light used.

Answer: It means that the speed of light in diamond is 1/2.42 of its speed in vacuum, that is, light travels 2.42 times slower in diamond than in air or vacuum. Since n = c/v, v = (3 × 10⁸)/2.42 ≈ 1.24 × 10⁸ m/s.

Teacher's Note:
1. Refractive index is the ratio of the speed of light in vacuum to its speed in the medium.
2. A higher refractive index means the medium is optically denser.

Answer: Refraction is the change in the direction of light when it passes obliquely from one transparent medium to another. Laws: (1) The incident ray, the refracted ray and the normal at the point of incidence all lie in the same plane. (2) sin i / sin r is constant for a given pair of media (Snell's law).

Teacher's Note:
1. Refraction occurs because the speed of light is different in different media.
2. A ray entering normally does not bend.

Answer: A real image is formed when reflected or refracted rays actually meet; it can be obtained on a screen and is inverted. A virtual image is formed when the rays only appear to meet when extended backwards; it cannot be obtained on a screen and is erect.

Teacher's Note:
1. A plane mirror always forms a virtual image.
2. A concave mirror can form both real and virtual images.

Answer: The person cannot see distant objects, so the defect is myopia (short-sightedness). A concave lens must form the image of a distant object at the far point, so u = −∞ and v = −80 cm, giving f = −80 cm = −0.8 m.
Power P = 1/f = 1/(−0.8) = −1.25 D.

Teacher's Note:
1. A concave lens of negative power diverges the rays so that they appear to come from the far point.
2. Remember to write the negative sign with the power.

Answer: Starlight passes through layers of the atmosphere whose refractive index keeps changing, so the apparent position and brightness of the star vary from moment to moment. A star is a point source, so this change is noticeable. A planet is much closer and acts as an extended source made of many point sources; the variations average out and it does not twinkle.

Teacher's Note:
1. Twinkling is caused by atmospheric refraction.
2. Stars would not twinkle if seen from outside the atmosphere.

Answer: The retina has rod and cone cells that are sensitive to light. They generate electrical signals when light falls on them, and these signals are carried by the optic nerve to the brain. The brain interprets the signals and we see an erect image of the object.

Teacher's Note:
1. The image on the retina is real and inverted.
2. The brain processes the signals and corrects the orientation.

Answer: (a) Cornea: the transparent front part of the eye; most of the refraction of light entering the eye takes place here.
(b) Pupil: regulates the amount of light entering the eye.
(c) Ciliary muscles: change the curvature (focal length) of the eye lens so that near and distant objects can be focused on the retina.

Teacher's Note:
1. The change in the focal length of the eye lens is called accommodation.
2. The eye lens only fine-tunes the focusing; the cornea does most of the bending.