SPI Physics · Free notes

Light and Optics: SPI Physics

Light and Optics has 16 questions in the SPI bank, eleven EASY and five MODERATE, all from sample papers and mock tests. Most ask one fact: which mirror a car uses, which lens fixes a defect of the eye, what a prism does to white light. The MODERATE ones ask a reason instead, such as why a lens is weaker in water or where a convex lens gives an image as far away as the object. These pages give the facts in short tables and the reasons in a few plain rules about how light bends.

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.

SPI home All formulas on one page

Subtopic notes

Formula & revision sheet

3 formulas · 5 reference tables · 18 gotchas across all subtopics — the exam-eve cheat-sheet

Light and Reflection

Formulas (1)

The laws of reflection · Law of reflection $\angle r = \angle i \qquad \angle i = 90^\circ - (\text{angle between the ray and the mirror})$

Reference tables (2)

What light is7 rows

Fact	Value
Kind of wave	Electromagnetic
Transverse or longitudinal	Transverse Sound is the longitudinal one; light is not.
Needs a medium	No, it travels through a vacuum
Speed in a vacuum	$3 \times 10^8$ m/s
Path in one medium	Straight lines, so opaque objects cast shadows
Ray	One straight line along which light travels
Beam	A bundle of rays

Radio waves, X-rays and light are all electromagnetic waves; they differ only in wavelength.

Plane, concave and convex mirrors and their uses4 rows

Mirror	Image	Uses
Plane	Virtual, erect, same size	Looking glass, periscope
Concave, object far away	Real and inverted	Torch and headlight reflectors, solar cookers
Concave, object very close	Virtual, erect, enlarged	Shaving and make-up mirrors, a dentist's mirror
Convex	Always virtual, erect, diminished	Rear-view and side-view mirrors of vehicles, shop security mirrors Widest field of view of the three.

A torch or headlight puts its bulb at the focus of a concave mirror to send out a parallel beam.

Watch out for (6)

Calling light a radio wave
→ What light is
Calling light longitudinal
→ What light is
Measuring from the mirror
→ The laws of reflection
Thinking the law fails for curved mirrors
→ The laws of reflection
Choosing a plane mirror for the rear view
→ Plane, concave and convex mirrors and their uses
Thinking a concave mirror always gives a real image
→ Plane, concave and convex mirrors and their uses

Refraction and Lenses

Formulas (2)

Refractive index and the bending of light · Refractive index $n = \frac{c}{v} \qquad n_{21} = \frac{n_2}{n_1} = \frac{v_1}{v_2}$
Convex and concave lenses, the lens formula and power · Lens formula, magnification and power $\frac{1}{v} - \frac{1}{u} = \frac{1}{f} \qquad m = \frac{v}{u} \qquad P = \frac{1}{f\,(\text{m})}$

Watch out for (6)

Higher refractive index means faster light
→ Refractive index and the bending of light
Thinking water makes a lens stronger
→ Refractive index and the bending of light
Picking the pair with the largest indices
→ Refractive index and the bending of light
Using centimetres in the power formula
→ Convex and concave lenses, the lens formula and power
Forgetting the sign of a concave lens
→ Convex and concave lenses, the lens formula and power
Putting the same-size image at F
→ Convex and concave lenses, the lens formula and power

The Eye, Dispersion and Instruments

Reference tables (3)

Defects of vision and the lens that corrects each5 rows

Defect	What goes wrong	Corrected with
Myopia (short sight)	Far objects blur; the image forms in front of the retina	Concave lens
Hypermetropia (long sight)	Near objects blur; the image forms behind the retina	Convex lens
Presbyopia	Old age: the ciliary muscles weaken, so near and often far vision both suffer	Bifocal lens
Astigmatism	The cornea is curved unequally, so lines in one direction blur	Cylindrical lens Not convex or concave: a cylindrical lens.
Cataract	The eye lens turns cloudy	Surgery to replace the lens

Short sight takes a concave lens, long sight a convex one: the opposite of what the names might suggest.

Dispersion, the spectrum and atmospheric refraction7 rows

Event	Cause
A prism forms a spectrum	Dispersion: each colour refracts by a different amount
A rainbow	Dispersion and reflection inside raindrops; seen with the Sun behind you
Stars twinkle	Atmospheric refraction that keeps changing
Planets do not twinkle	They are close, so they look like discs, and the changes average out
The Sun is seen about 2 minutes before it rises	Atmospheric refraction bends its light over the horizon
The sky is blue	Air scatters blue light much more than red
Danger signals are red	Red is scattered least, so it is seen from far away

Splitting is dispersion; bending is refraction; spreading by tiny particles is scattering.

Optical instruments and what each is for7 rows

Instrument	Used for
Telescope	Distant objects such as stars and planets
Binoculars	Distant objects, with both eyes
Microscope	Very small objects close by, such as cells
Periscope	Seeing over an obstacle, using two plane mirrors
Kaleidoscope	Repeating patterns from plane mirrors
Spectroscope	Splitting light into its spectrum to study it
Chronometer	Measuring time very accurately Not an optical instrument at all.

The suffix tells the job: tele means far, micro means small, peri means around.

Watch out for (6)

Concave lens for long sight
→ Defects of vision and the lens that corrects each
Bifocal for astigmatism
→ Defects of vision and the lens that corrects each
Calling the splitting 'refraction'
→ Dispersion, the spectrum and atmospheric refraction
Violet bends least
→ Dispersion, the spectrum and atmospheric refraction
Planets twinkle too
→ Dispersion, the spectrum and atmospheric refraction
Telescope and microscope swapped
→ Optical instruments and what each is for

Question weightage by concept

8 concepts · 16 questions — where the marks actually sit, so you know what to drill first

Light and Reflection6 questions · 38%

Concept	Qs	Share
What light is	3	19%
Plane, concave and convex mirrors and their uses	2	13%
The laws of reflection	1	6%

Refraction and Lenses4 questions · 25%

Concept	Qs	Share
Refractive index and the bending of light	3	19%
Convex and concave lenses, the lens formula and power	1	6%

The Eye, Dispersion and Instruments6 questions · 38%

Concept	Qs	Share
Dispersion, the spectrum and atmospheric refraction	3	19%
Defects of vision and the lens that corrects each	2	13%
Optical instruments and what each is for	1	6%

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