Class 10 Science Chapter 10: The Human Eye and the Colourful World — Important Questions & Sample Paper

45°

Light is deviated but no dispersion occurs

Myopia

−0.5 D

Both A and R are true but R is not the correct explanation of A.

On Earth, the atmosphere scatters sunlight, with blue light being scattered the most, giving the sky a blue colour. In space, there is no atmosphere, hence no scattering of light, making the sky appear dark to an astronaut.

The corrective lens should have a focal length such that an object at infinity appears to come from the far point (80 cm). So, f = -80 cm = -0.8 m. Power P = 1/f = 1/(-0.8) = -1.25 D. Hence, a concave lens of power -1.25 D is needed.

The colour of an object is determined by the wavelengths of light it reflects. A red rose appears red because it reflects mainly red light and absorbs other colours. In white light (containing all colours), it reflects red, so it looks red. When illuminated by pure blue light, there is no red light present in the incident light. The rose absorbs the blue light and reflects nothing (or very little), so it appears black (absence of reflected visible light).

He suffers from myopia (nearsightedness). To correct it, a concave lens is used. For a distant object (at infinity), the lens must form its image at the person’s far point (2 m). Using lens formula, with u = –∞, v = –2 m (virtual image on same side as object), we get 1/f = 1/v – 1/u = –1/2 – 0 = –0.5 m⁻¹. So f = –2 m. Power P = 1/f (in m) = –0.5 D. Thus, a concave lens of power –0.5 D is needed.

Power P = +2 D = 1/f (in meters) => f = 1/P = 1/2 = 0.5 m = 50 cm (convex lens). The defect is hypermetropia (far-sightedness). One cause: the eyeball is too short, so the retina is closer to the lens than normal, causing light from near objects to focus behind the retina. Correction: a convex lens of suitable power is placed in front of the eye. The rays from a near object (at 25 cm) first pass through the convex lens which converges them, and then the eye lens further converges them to form a sharp image on the retina. Ray diagram: Without correction, rays from near point (farther) focus behind retina; with convex lens, rays from 25 cm are made to appear coming from the person's near point, so eye sees clear image.

The defect is hypermetropia (long-sightedness). Causes: (1) The eyeball is too short from front to back, (2) The eye lens is too flat, resulting in low converging power. The near point is farther than 25 cm. Light from a near object focuses behind the retina. Correction: a convex lens (converging lens) of appropriate power is placed in front of the eye. The convex lens first converges the incoming light slightly before it enters the eye, allowing the eye lens to then focus the image exactly on the retina. Ray diagram: (without correction) parallel rays from near object diverge after eye lens, converging behind retina; (with correction) convex lens converges rays, then eye lens focuses on retina.

Rayleigh scattering: I ∝ 1/λ⁴. Violet (400 nm) is scattered (700/400)⁴ ≈ 9.4 times more than red (700 nm). In space, absence of atmospheric particles means no scattering, making the sky dark.