(a) (i) (1) What are coherent sources ? Why are they necessary for observing a sustained interference pattern ?\n(2) Lights from two independent sources are not coherent. Explain.\n(ii) Two slits $0.1\text{ mm}$ apart are arranged $1.20\text{ m}$ from a screen. Light of wavelength $600\text{ nm}$ from a distant source is incident on the slits.\n(1) How far apart will adjacent bright interference fringes be on the screen ?\n(2) Find the angular width (in degree) of the first bright fringe.\nOR\n(b) (i) Define a wavefront. An incident plane wave falls on a convex lens and gets refracted through it. Draw a diagram to show the incident and refracted wavefront.\n(ii) A beam of light coming from a distant source is refracted by a spherical glass ball (refractive index 1.5) of radius $15\text{ cm}$. Draw the ray diagram and obtain the position of the final image formed.

Year	Paper Code	Set	Position
CBSE 2025	Series 55-1	Set 1	Question 33
CBSE 2025	Series 55-1	Set 2	Question 31
CBSE 2025	Series 55-1	Set 3	Question 31

• Step 1: Stating the fundamental formula / law statement (+1 Mark)
• Step 2: Intermediate mathematical substitution or proof steps (+1 Mark)
• Step 3: Correct final value or conclusion with mandatory SI units (+1 Mark)