Light – Reflection and Refraction — Important Questions

Convex lens

Step 1: A convex lens is also called a converging lens because it converges light rays. Step 2: Its shape is thicker at the center and thinner at the edges, like a lentil seed. Step 3: This shape allows it to bend light rays inward toward the principal axis. Step 4: This is why convex lenses are used in magnifying glasses - they converge light to form enlarged images. Step 5: Concave lenses are thinner at the center and thicker at the edges. There's no such thing as a plane lens in optics.

Virtual, erect, and diminished

Step 1: A concave lens is a diverging lens that spreads out light rays. Step 2: No matter where you place an object in front of a concave lens, it always forms the same type of image. Step 3: The image is always virtual (cannot be obtained on screen), erect (upright), and diminished (smaller than object). Step 4: This is because the diverging rays appear to come from a point in front of the lens when traced backward. Step 5: This property makes concave lenses useful for correcting nearsightedness in spectacles.

Dioptres

Step 1: The power of a lens is defined as the reciprocal of its focal length (P = 1/f). Step 2: When focal length is measured in meters, power is measured in dioptres. Step 3: The symbol for dioptre is 'D', and 1D = 1/meter = 1 m⁻¹. Step 4: Opticians use dioptres to prescribe spectacles - for example, +2D means a convex lens with focal length 0.5m. Step 5: Meters and centimeters are units of length, not power. Degrees measure angles, not lens power.

Refraction

Step 1: Initially, the coin is not visible because light from the coin cannot reach your eyes due to the bowl's sides blocking the path. Step 2: When water is poured, light from the coin travels through water and then into air. Step 3: At the water-air interface, light rays bend (refract) due to the change in medium density. Step 4: This bending allows the light rays to reach your eyes, making the coin visible. Step 5: The coin appears raised from its actual position due to refraction. This is not reflection (bouncing back), diffraction (bending around obstacles), or absorption (taking in ligh