CBSE Class 10 Physics Light Reflection and Refraction Notes Set 02

Focal Length of a Concave Mirror

1. Distant-object method: focus the image of a far-off object (a tree, a window) sharply on a screen. The distance from the mirror to the screen is the approximate focal length f.
2. u-v method: place a bright pin at several distances u in front of the mirror, find the position v of its real inverted image by removing parallax with a second pin, and calculate f from 1/f = 1/u + 1/v for each pair (magnitudes). Take the mean.
3. Normal reflection method: move the pin until its inverted image falls exactly on the pin itself. The pin is then at the centre of curvature, so the distance to the mirror is R and f = R/2.
4. Graphs: a plot of 1/u against 1/v is a straight line with equal intercepts on both axes, each equal to 1/f. A plot of v against u meets the line v = u at the point (2f, 2f).

Refraction Through a Glass Slab

1. Place the slab on paper, draw its outline, fix two pins P\( _1 \) and P\( _2 \) on the incident ray and look through the opposite face to fix P\( _3 \) and P\( _4 \) so that all four pins appear in one line. Join P\( _3 \)P\( _4 \), draw the normals and measure the angles i and r.
2. The laws of refraction are verified: the incident ray, refracted ray and normal lie in one plane, and sin i / sin r is constant. For ordinary glass \( \mu \) = sin i / sin r is about 1.5.
3. The emergent ray is parallel to the incident ray and is shifted sideways. The lateral shift increases with the thickness of the slab and the angle of incidence.
4. A graph of sin i against sin r is a straight line through the origin, and its slope is the refractive index.

Refraction Through a Prism

1. Trace the path of a ray through the prism for angles of incidence from 35\( ^\circ \) to 65\( ^\circ \) and measure the deviation d. The graph of d against i dips to a minimum value D, which for a 60\( ^\circ \) glass prism is about 38\( ^\circ \), at i about 49\( ^\circ \).
2. The refractive index of the prism is \( \mu \) = sin[(A + D)/2] / sin(A/2), which comes out near 1.5.
3. Critical angle: when light goes from glass to air at the critical angle C, the refracted ray grazes the surface. For glass C is about 42\( ^\circ \), and \( \mu \) = 1/sin C. For angles of incidence greater than C the light is totally reflected.

Focal Length of a Convex Lens

1. Distant-object method: focus a far-off object on a screen and measure the lens-to-screen distance (approximate).
2. u-v method: for several values of u, find v and use 1/f = 1/v \( - \) 1/u (with signs), or use magnitudes. When u = v, the object is at 2f, and f = u/2. The least distance between a real object and its real image is 4f.
3. Graphs: v against u meets the line v = u at (2f, 2f); 1/v against 1/u is a straight line with intercepts 1/f on both axes.