Ray Optics and Optical Instruments for NEET UG
Treat light as rays and you can explain mirrors, lenses, prisms, the human eye and microscopes. Nearly every NEET optics problem uses one of three formulas.
PhysicsClass 1264 MCQs3 lessons13 flashcards4 memory tricks
Practise Ray Optics and Optical Instruments free
What you'll learn
- Mirrors and the mirror formula
Use the sign convention and the mirror formula to locate images. - Refraction, total internal reflection and prisms
Use Snell's law, find the critical angle, and calculate deviation by a prism. - Lenses, the eye and optical instruments
Use the lens formula and power, correct eye defects, and find the magnification of a microscope and telescope.
Key formulas
| Mirror formula | 1/v + 1/u = 1/f |
|---|---|
| Focal length | f = R/2 |
| Magnification | m = −v/u = h'/h |
| Snell's law | n₁ sin i = n₂ sin r |
| Refractive index | n = c/v |
| Critical angle | sin C = 1/n (into air) |
| Apparent depth | apparent depth = real depth/n |
| Prism (minimum deviation) | n = sin[(A + δ_m)/2]/sin(A/2) |
Memory tricks
Red deviates least and has the longest wavelength. Violet deviates most and has the shortest wavelength.
Total internal reflection happens when light goes from a denser to a rarer medium at more than the critical angle (sin C = 1/μ). It's used in optical fibres, and it's why diamonds sparkle.
Short sight (myopia) is corrected with a concave (diverging, negative power) lens; long sight (hypermetropia) with a convex (positive) lens. P = 1/f with f in metres.
Apparent depth = real depth ÷ μ. Water (μ ≈ 4/3) makes a 2 m pool look only 1.5 m deep.
Try 3 NEET-style questions
Q1. Convex mirrors are used as rear-view mirrors in vehicles mainly because they
- form real, magnified images
- form inverted images that are easy to judge
- concentrate light to a sharp focus
- give an erect image over a wide field of view
Show answer
Answer: (4) give an erect image over a wide field of view
A convex mirror always forms a virtual, erect, diminished image, which lets the driver see a wide area behind.
Q2. The objective lens of an astronomical telescope has a large diameter (aperture) mainly to
- increase the magnifying power only
- gather more light and improve resolving power
- reduce the focal length
- make the image virtual
Show answer
Answer: (2) gather more light and improve resolving power
A large aperture collects more light (brighter images of faint objects) and improves resolution (ability to separate close objects).
Q3. A thin prism of angle 5° is made of glass of refractive index 1.6. The deviation it produces is
- 8°
- 5°
- 3°
- 0.6°
Show answer
Answer: (3) 3°
For a thin prism δ = (n − 1)A = 0.6 × 5° = 3°.
Practise all 64 questions Free · explanations for every option · see where you go wrong