Physics notes · Chapter 8 of 11
Light & Optics
15 sections, 68 flashcards. Open a card to check your answer, or revise them in the app with spaced revision.
Nature of Light
- Light is a form of energy our eyes can detect; without it there is no vision.
- It travels in straight lines (rectilinear propagation), which is why objects cast sharp shadows.
- Its speed in vacuum is about 3 × 10⁸ m/s (3 lakh km/s), the fastest thing in the universe; sunlight takes about 8 minutes 20 seconds to reach the Earth.
- Dual nature: light behaves both as a wave and as particles called photons.
Check yourself
Define light
A form of energy that our eyes can detect; without light there is no vision.
How does light travel?
In straight lines (rectilinear propagation) — which is why objects cast sharp shadows.
Speed of light in vacuum
About 3 × 10⁸ m/s (3 lakh km/s) — the fastest thing in the universe.
What is the dual nature of light?
It behaves both as a wave and as particles called photons.
How long does sunlight take to reach Earth?
About 8 minutes 20 seconds.
Visible Light & VIBGYOR
- White light is seven colours, VIBGYOR: violet, indigo, blue, green, yellow, orange, red.
- Visible wavelengths run from about 4000 Å (violet) to 7000 Å (red) (1 Å = 10⁻¹⁰ m).
- Violet: shortest wavelength, highest frequency (~7.5 × 10¹⁴ Hz). Red: longest wavelength, lowest frequency (~4 × 10¹⁴ Hz).
- Colour, Wavelength, Frequency
| Colour | Wavelength | Frequency |
|---|---|---|
| Violet | Shortest | Highest |
| Red | Longest | Lowest |
Check yourself
What is white light made of?
Seven colours – VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red.
Range of visible wavelengths
About 4000 Å (violet) to 7000 Å (red), where 1 Å = 10⁻¹⁰ m.
Violet: wavelength and frequency
Shortest wavelength, highest frequency (~7.5 × 10¹⁴ Hz).
Red: wavelength and frequency
Longest wavelength, lowest frequency (~4 × 10¹⁴ Hz).
Colour of light is determined by…
Its wavelength (frequency) — not its speed or intensity.
Electromagnetic Spectrum
- The EM spectrum, low to high energy: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Gamma rays carry the most energy; energy rises as wavelength falls.
- All EM waves travel at the same speed in vacuum, 3 × 10⁸ m/s.
- Crookes glass (UV-filtering glass) cuts off ultraviolet rays.
- EM Waves by Energy
| Wave | Energy |
|---|---|
| Radio | Lowest |
| Microwave | Low |
| Infrared / Visible | Medium |
| Ultraviolet | High |
| X-ray | Higher |
| Gamma | Maximum |
Check yourself
Full EM spectrum, low to high energy
Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
Which carries maximum energy?
Gamma rays — energy increases as wavelength decreases.
Do all EM waves travel at the same speed?
Yes — 3 × 10⁸ m/s in vacuum.
Which glass cuts off UV rays?
Crookes glass / ultraviolet-filtering glass.
Reflection of Light
- Reflection: light bouncing back from a surface. The normal is the line perpendicular to the surface where the light strikes.
- First law: angle of incidence = angle of reflection (i = r). Second law: the incident ray, reflected ray and normal lie in the same plane.
- The laws hold for plane and curved (spherical) mirrors alike.
Check yourself
Define reflection
The bouncing back of light when it strikes a surface.
Define the normal
The line drawn perpendicular (90°) to the surface at the point of incidence.
First law of reflection
Angle of incidence equals angle of reflection (i = r).
Second law of reflection
The incident ray, reflected ray and normal lie in the same plane.
Do these laws hold for curved mirrors?
Yes — for both plane and curved (spherical) mirrors.
Real vs Virtual Image
- Real image: rays actually meet; it can be caught on a screen and is usually inverted (a cinema screen).
- Virtual image: rays only appear to meet; it cannot be caught on a screen and is erect (a bathroom mirror).
- Real vs Virtual Image
| Property | Real | Virtual |
|---|---|---|
| On screen | Yes | No |
| Orientation | Inverted | Erect |
| Where rays meet | Actually | Appear to |
Check yourself
Define a real image
Forms where rays actually meet; can be caught on a screen and is usually inverted.
Define a virtual image
Forms where rays only appear to meet; cannot be caught on a screen and is erect.
Cinema screen vs bathroom mirror
Cinema: real, inverted image from a projector. Mirror: virtual, erect image.
Plane (Flat) Mirror
- A plane mirror's image is virtual, erect, the same size, and as far behind the mirror as the object is in front.
- Lateral inversion: left and right are swapped, which is why AMBULANCE is printed reversed.
Its focal length is infinite (infinite radius of curvature).
To see your full image you need a mirror half your height.
Check yourself
Image in a plane mirror is…
Virtual, erect, same size, and as far behind as the object is in front.
Define lateral inversion
Left and right are swapped (e.g. AMBULANCE printed reversed).
Are top and bottom flipped in a plane mirror?
No — only sides are swapped; the image is not upside-down.
Focal length of a plane mirror
Infinity — it has an infinite radius of curvature.
Minimum mirror height to see your full image
Half your height.
Spherical Mirrors & Their Terms
- A concave mirror curves inward and converges light; a convex mirror bulges outward and diverges it.
- Pole (P): the centre of the mirror's surface. Centre of curvature (C): the centre of its sphere. Principal focus (F): where parallel rays meet (concave) or seem to come from (convex).
- f = R/2: the focal length is half the radius of curvature.
- Mirror Types
| Mirror | Shape | Action |
|---|---|---|
| Concave | Curves in | Converging |
| Convex | Bulges out | Diverging |
Check yourself
Concave vs convex mirror
Concave curves inward and is converging; convex bulges outward and is diverging.
Define pole (P) and centre of curvature (C)
P is the centre of the mirror surface; C is the sphere's centre.
Define principal focus (F)
Where parallel rays meet (concave) or seem to come from (convex).
Key mirror relation
Focal length is half the radius of curvature, f = R/2.
Images by Concave Mirror & Its Uses
- A concave mirror magnifies in a shaving or make-up mirror and a dentist's mirror, and reflects light in torches, headlights and solar cookers.
A convex mirror's image is always small, erect and virtual: hence its use as a rear-view mirror.
- Concave Mirror: Object Position vs Image
| Object position | Image |
|---|---|
| At C | Real, inverted, same size |
| Between P & F | Virtual, erect, magnified |
| Beyond C | Real, inverted, smaller |
Check yourself
Object at C — what image?
Real, inverted and the same size, also at C.
Object between P and F — what image?
Virtual, erect, magnified.
What uses the magnifying behaviour?
A shaving / make-up mirror and a dentist's mirror.
Other uses of concave mirrors
Reflectors in torches and headlights and in solar cookers.
Image in a convex mirror is always…
Small, erect, virtual — used as a vehicle rear-view mirror.
Refraction of Light
- Refraction: light bending as it passes from one medium to another, because its speed changes; a pencil in a glass of water looks bent.
- Light is slowest in the densest medium (glass, diamond) and fastest in vacuum. The refractive index measures how much a medium slows and bends it.
Check yourself
Define refraction
The bending of light as it passes from one medium to another.
Why does light bend?
Its speed changes between media of different density.
Where is the speed of light minimum?
In the densest medium — slowest in glass/diamond, fastest in vacuum.
What does NOT change during refraction?
The frequency (and colour) — only speed and wavelength change.
Define refractive index
Measures how much a medium slows and bends light.
Everyday example of refraction
A pencil in a glass of water looks bent.
Total Internal Reflection
- Total internal reflection: light in a dense medium meeting the boundary beyond the critical angle is completely reflected back inside instead of refracting out.
- A cut diamond sparkles through repeated total internal reflection (refractive index ≈ 2.42); optical fibres use it to carry light signals over long distances.
Check yourself
Define total internal reflection (TIR)
Occurs when light in a dense medium hits the boundary beyond the critical angle.
What happens instead of refracting out?
The light is completely reflected back inside.
Why does a cut diamond sparkle?
Because of repeated total internal reflection (refractive index ≈ 2.42).
How do optical fibres work?
They transmit light signals over long distances using TIR.
What causes a mirage on hot roads?
Refraction and total internal reflection in air layers.
Lenses
- A convex (converging) lens is thick in the middle and brings rays to a focus; a concave (diverging) lens is thin in the middle and spreads them.
- A magnifying glass is a convex lens, giving an enlarged, erect, virtual image.
- Lens power is in dioptres (D): positive for convex, negative for concave.
- Lens Types
| Lens | Centre | Action |
|---|---|---|
| Convex | Thick | Converging |
| Concave | Thin | Diverging |
Check yourself
Convex vs concave lens
Convex (converging) is thick in the middle and brings rays to a focus; concave (diverging) is thin in the middle and spreads rays apart.
Which lens is a magnifying glass?
A convex lens — gives an enlarged, erect, virtual image.
Covering part of a convex lens with black strips does what?
Only dims the image; the whole image still forms.
Unit of lens power and its sign
Dioptres (D); convex power is positive, concave negative.
Human Eye & Defects of Vision
- A normal eye's least distance of distinct vision is about 25 cm.
- Myopia (near-sightedness: near is clear, far is not) is corrected with a concave lens; hypermetropia (far-sightedness) with a convex lens.
- Presbyopia (sight lost with age) needs bifocal lenses; a cataract clouds the eye's lens.
- Defects of Vision
| Defect | Problem | Correction |
|---|---|---|
| Myopia | Can't see far | Concave lens |
| Hypermetropia | Can't see near | Convex lens |
| Presbyopia | Both | Bifocal lens |
Check yourself
Least distance of distinct vision
About 25 cm for a normal eye.
Define myopia and its correction
Near-sightedness — sees nearby objects clearly but not distant ones; corrected with a concave (diverging) lens.
Define hypermetropia and its correction
Far-sightedness — corrected using a convex (converging) lens.
Presbyopia and cataract
Presbyopia (old-age sight loss) needs bifocal lenses; cataract clouds the lens.
Dispersion & Colour Mixing
- Dispersion: a prism splitting white light into seven colours. A rainbow forms by dispersion, refraction and reflection of sunlight in raindrops.
- Primary colours of light: red, green and blue. Additive mixing: all three make white; red + green = yellow; green + blue = cyan; red + blue = magenta.
- Soap bubbles and oil films get their colours from thin-film interference.
- Additive Colour Mixing
| Colours mixed | Result |
|---|---|
| Red + Green | Yellow |
| Green + Blue | Cyan |
| Red + Blue | Magenta |
| Red+Green+Blue | White |
Check yourself
Define dispersion
The splitting of white light into seven colours by a prism.
How does a rainbow form?
By dispersion, refraction and reflection of sunlight in raindrops.
Primary colours of light
Red, green and blue (additive mixing).
Additive mixing results
R+G+B = white; red+green = yellow; green+blue = cyan; red+blue = magenta.
What colours soap bubbles and oil films?
Thin-film interference.
Scattering & Atmospheric Effects
- Scattering: air molecules and dust spread (diffuse) sunlight in the atmosphere; diffusion of light is mainly due to scattering by dust.
The Sun looks red at sunrise and sunset because the blue is scattered away.
Stars twinkle because of atmospheric refraction.
Check yourself
Define scattering of light
Air molecules spread (diffuse) sunlight in the atmosphere.
Why does the sky look blue?
Shorter blue wavelengths scatter the most (Rayleigh scattering).
Why does the Sun look red at sunrise/sunset?
Blue light scatters away and red reaches the eye.
What causes the twinkling of stars?
Atmospheric refraction of starlight.
Diffusion of light in the atmosphere is due to…
Scattering (mainly by dust particles).
Optical Instruments & Resolution
- A telescope views distant objects; a microscope magnifies tiny ones.
- Resolution: the ability to see two close objects as separate.
Check yourself
Telescope vs microscope
A telescope views distant objects; a microscope magnifies tiny objects.
Larger telescope objective diameter does what?
Increases resolving power and gathers more light, giving brighter, clearer images.
Define resolution
The ability to see two close objects as separate.
All 11 chapters of Physics notes
- Units, Measurement & Physical Quantities11 sections
- Motion, Laws of Motion & Forces13 sections
- Gravitation11 sections
- Work, Power, Energy & Conservation13 sections
- Properties of Matter & Fluids12 sections
- Heat & Thermodynamics12 sections
- Waves & Sound11 sections
- Light & Optics15 sections
- Current Electricity13 sections
- Magnetism & Electromagnetism15 sections
- Modern & Nuclear Physics16 sections