The Human Eye and the Colourful World Practice

Follow this pattern for 5 marks: (1) Name the defect. (2) State what the person can and cannot see clearly. (3) State where the image forms (in front of or behind the retina). (4) Give two causes. (5) State the corrective lens. (6) Draw a neat labelled ray diagram showing the defect and the correction.

They are the same thing. The near point of a normal human eye is about 25 cm, which is also called the least distance of distinct vision (D). Objects closer than this appear blurred because the eye's ciliary muscles cannot increase the lens curvature further.

The eye lens must change its focal length rapidly to focus on objects at different distances — this is the power of accommodation. If the lens were rigid (as happens with age in presbyopia), it could only focus at one fixed distance. Flexibility, controlled by ciliary muscles, allows the same lens to serve near and far vision.

Yes. As a person ages, they may develop presbyopia along with pre-existing myopia or hypermetropia. In such cases, bifocal spectacles are prescribed: the upper portion corrects distance vision (concave for myopic) and the lower portion corrects near vision (convex for near reading). Progressive lenses provide a gradual change in power.

VIBGYOR stands for Violet, Indigo, Blue, Green, Yellow, Orange, Red — the seven colours of the visible spectrum. Violet bends (deviates) the most in a prism because it has the shortest wavelength and the highest refractive index in glass. Red bends the least because it has the longest wavelength.

The Tyndall effect requires particles that are large enough to scatter visible light (particle size comparable to wavelength of light, i.e., about 400–700 nm). In a true solution like salt water, the dissolved ions are only a few angstroms in size — far too small to scatter visible light effectively. So the path of a light beam is not visible in a true solution. It is visible in colloidal solutions or suspensions.

Due to atmospheric refraction, light from a star just below the horizon bends towards the Earth's surface as it travels through increasingly dense air layers. This bending makes the star appear to be above the horizon even when it is geometrically below it. The star's apparent position is thus shifted upward from its actual position.