Refraction, Lenses and the Eye
NCERT Class 10 • Science • Chapter "Light Reflection and Refraction" | NCERT Class 10 • Science • Chapter "The Human Eye and the Colourful World"
Refraction, Lenses and the Eye
Refraction is the bending of light when it passes from one transparent medium into another because its speed changes. This single effect explains why a pencil looks bent in water, why a pool looks shallower than it is, and how lenses form images.
Refraction and Lenses
- Light bends towards the normal when it goes from a rarer to a denser medium and away from the normal in the reverse case.
- Refractive index compares the speed of light in vacuum with its speed in the medium; a higher index means greater bending.
- Total internal reflection occurs when light in a denser medium meets the boundary beyond the critical angle, and it makes optical fibres and mirages possible.
- A convex or converging lens is thicker in the middle and brings parallel rays to a focus.
- A concave or diverging lens is thinner in the middle and spreads parallel rays out.
- The power of a lens is the reciprocal of its focal length in metres and is measured in dioptre.
- Power is positive for a convex lens and negative for a concave lens.
The Human Eye and Its Defects
- The cornea admits light, the iris controls the size of the pupil, and the eye lens focuses light on the retina.
- The retina carries light sensitive cells that send signals to the brain through the optic nerve.
- Accommodation is the ability of the eye lens to change its focal length so that both near and distant objects can be seen clearly.
- Myopia or short sightedness means distant objects look blurred; the image forms in front of the retina and it is corrected with a concave lens.
- Hypermetropia or long sightedness means near objects look blurred; the image forms behind the retina and it is corrected with a convex lens.
- Presbyopia comes with age as the eye lens loses flexibility, and cataract is the clouding of the lens.
- Dispersion of white light through a prism gives the seven colours, and the same effect in water droplets forms a rainbow.
| Defect | Image Forms | Corrective Lens |
|---|---|---|
| Myopia | In front of retina | Concave |
| Hypermetropia | Behind retina | Convex |
| Presbyopia | Loss of accommodation with age | Bifocal |
Power of a lens — the reciprocal of the focal length in metres, measured in dioptre, showing how strongly the lens bends light.
Exam me kaise aata hai
- Myopia is corrected using a — concave lens
- Hypermetropia is corrected using a — convex lens
- The unit of power of a lens is the — dioptre
- Light focuses on which part of the eye — the retina
UPSC/State PSC ke liye
- Total internal reflection is why an optical fibre can carry light around bends with almost no loss, which is the basis of modern communication cables.
- The blue colour of the sky and the red of the setting sun both come from scattering, where shorter wavelengths scatter far more than longer ones.
Yahan confuse hote hain
✗ Myopia is corrected with a convex lens | Myopia needs a concave lens | ✓
✗ A concave lens has positive power | Concave lens power is negative | ✓
✗ Refraction happens because light slows down only in water | It happens whenever the speed of light changes between media | ✓
Ek nazar me
- Refraction is bending due to a change in the speed of light.
- Convex lens converges with positive power; concave diverges with negative power.
- The eye focuses on the retina and adjusts by accommodation.
- Myopia needs a concave lens and hypermetropia a convex lens.