IMAT Physics · Free notes

Optics: Mirrors, Refraction, Lenses and the Eye

Optics has a single past question since 2011, on a concave mirror, and the ministry papers from 2023 on have not asked about it at all. That makes it one of the least tested chapters, but the syllabus still lists it, and the eye, the endoscope and corrective lenses make it natural material for a medical admission test. Expect either a fact to recall (which lens corrects short sight, when light is totally reflected) or a short calculation with Snell's law or the thin lens equation. The traps are in the conventions: angles from the normal, focal lengths in metres for dioptres, and the sign that marks an image as virtual.

The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.

IMAT home All formulas on one page

Subtopic notes

Formula & revision sheet

4 formulas · 3 reference tables · 8 gotchas across all subtopics — the exam-eve cheat-sheet

Mirrors, Refraction and Total Internal Reflection

Formulas (2)

Refraction, refractive index and Snell's law · Refractive index and Snell's law $n = \frac{c}{v} \qquad n_1 \sin\theta_1 = n_2 \sin\theta_2$
Total internal reflection and the critical angle · Critical angle $\sin C = \frac{n_2}{n_1} \qquad \sin C = \frac{1}{n} \ \text{(into air)}$

Reference tables (2)

The laws of reflection and the plane mirror image5 rows

Property	Plane mirror image
Position	As far behind the mirror as the object is in front
Size	Same size as the object
Orientation	Upright, but left and right are swapped (laterally inverted)
Type	Virtual: it cannot be projected on a screen
Full-length mirror	Needs to be only half your height

Concave and convex mirrors: rays and images6 rows

Mirror and object position	Image	Use
Concave, object beyond $C$	Real, inverted, smaller, between $F$ and $C$	Telescope mirrors collecting light from far away
Concave, object at $C$	Real, inverted, same size, at $C$	Checking the radius of curvature in a lab
Concave, object between $C$ and $F$	Real, inverted, larger, beyond $C$	Projecting an enlarged image onto a screen
Concave, object at $F$	No image: the reflected rays are parallel	Torch and headlamp reflectors, with the bulb at $F$
Concave, object between $F$ and the mirror	Virtual, upright, larger, behind the mirror	Make-up and dentists' mirrors
Convex, any position	Virtual, upright, smaller, behind the mirror	Car wing mirrors and shop security mirrors

Watch out for (4)

Angles in optics are measured from the normal
→ The laws of reflection and the plane mirror image
A concave mirror does not always give a real, inverted image
→ Concave and convex mirrors: rays and images
Refraction changes the wavelength, not the frequency
→ Refraction, refractive index and Snell's law
Total internal reflection only happens going into a less dense medium
→ Total internal reflection and the critical angle

Lenses, Power and Vision Defects

Formulas (2)

Converging and diverging lenses and the thin lens equation · Thin lens equation and magnification $\frac{1}{f} = \frac{1}{d_o} + \frac{1}{d_i} \qquad m = -\frac{d_i}{d_o} = \frac{h_i}{h_o}$
Lens power in dioptres · Power of a lens $P = \frac{1}{f} \qquad P_{\text{total}} = P_1 + P_2$

Reference tables (1)

The human eye, short sight and long sight7 rows

Part or defect	What it does or what goes wrong	Note or correction
Cornea	Does most of the focusing, at the curved air to cornea surface	Its power is fixed
Lens and ciliary muscles	Fine focusing: the muscles change the lens's shape (accommodation)	The lens becomes fatter for near objects
Iris and pupil	The iris changes the size of the pupil to control how much light enters	The pupil narrows in bright light
Retina	Light-sensitive layer where a real, inverted image forms	The brain interprets the image upright
Myopia (short sight)	Eyeball too long or lens too strong: image in front of the retina	Diverging lens, negative power
Hypermetropia (long sight)	Eyeball too short or lens too weak: image behind the retina	Converging lens, positive power
Presbyopia	With age the lens stiffens and cannot accommodate for near objects	Converging reading glasses

Watch out for (4)

A converging lens does not always give a real image
→ Converging and diverging lenses and the thin lens equation
Power needs the focal length in metres
→ Lens power in dioptres
Short sight needs a diverging lens
→ The human eye, short sight and long sight
The cornea, not the lens, does most of the focusing
→ The human eye, short sight and long sight

PYQ weightage by concept

7 concepts · 0 PYQs — where the marks actually sit, so you know what to drill first

Mirrors, Refraction and Total Internal Reflection0 PYQs · 0%

Concept	PYQs	Share
The laws of reflection and the plane mirror imagefoundation	—	—
Concave and convex mirrors: rays and imagesfoundation	—	—
Refraction, refractive index and Snell's lawfoundation	—	—
Total internal reflection and the critical anglefoundation	—	—

Lenses, Power and Vision Defects0 PYQs · 0%

Concept	PYQs	Share
Converging and diverging lenses and the thin lens equationfoundation	—	—
Lens power in dioptresfoundation	—	—
The human eye, short sight and long sightfoundation	—	—