SSC CGL Physics · Light and Optics
Mirrors, Lenses and Refraction
Concave mirrors, ray diagrams and lens power, what stays the same when light is refracted, the eye's near point and pupil, diffraction, gamma rays and the secondary rainbow.
Why this matters
Eleven questions in the bank: one from the 2023 shifts and five each from the 2024 and 2025 shifts, ten of them EASY. Each asks one fact from school optics, so the two tables here cover them.
Concept 1 of 2: Mirrors and lenses
Definition
Mirrors:
- A concave mirror converges light, so solar furnaces and cookers use it. A convex mirror spreads light.
- Two rays are the least needed to locate an image.
Lenses:
- Power P = 1/f, f in metres; unit dioptre.
- An object at the focus of a convex lens gives an image at infinity on the other side.
Practise this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 1 · Light and Optics · Mirrors, Lenses and Refraction
Biconvex is a lens
Concept 2 of 2: Refraction, the eye and light's behaviour
Definition
Refraction:
- Frequency stays the same; speed and wavelength fall.
- Snell (Snellius) stated the law of refraction (1621).
The eye:
- The least distance of distinct vision is about 25 cm.
- The iris controls the pupil: in bright light it contracts it, so in a dim room you see little until the pupil widens.
Other behaviour:
- Diffraction: light bends round a very small obstacle or gap.
- Gamma rays have the shortest wavelengths (below 10^-3 nm).
- A secondary rainbow has its colours reversed.
Practise this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 2 · Light and Optics · Mirrors, Lenses and Refraction
The iris, not the cornea
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Mirrors and lenses3 rows
Watch out for (2)
- Biconvex is a lens→ Mirrors and lenses
- The iris, not the cornea→ Refraction, the eye and light's behaviour
Test yourself on a real paper
Sit a past SSC CGL paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.