Your Friendly Guide to Light!

Hey there! Welcome to the amazing world of Light. Ever wondered how glasses help people see, why diamonds sparkle so brightly, or how we get super-fast internet through tiny glass wires? It's all about Physics, and specifically, how light behaves. This chapter might seem like it has a lot of rules and diagrams, but don't worry! We're going to break it all down into simple, easy-to-understand parts. We'll use everyday examples to make sense of everything. By the end, you'll see the world around you in a whole new light! (Pun intended!)

Let's get started on our journey through reflection, refraction, lenses, and the cool wave-like nature of light.


1. What is Light? (A Quick Refresher)

Before we dive deep, let's remember what light is. Light is a form of energy that travels in waves. It's part of a huge family called the Electromagnetic (EM) Spectrum.

The Electromagnetic Spectrum

The EM spectrum includes everything from radio waves to gamma rays. Visible light is just the tiny part of this spectrum that our eyes can detect.

Think of it like a massive radio dial. You can tune into different stations (radio waves, microwaves, X-rays), but your eyes are only built to 'see' the small band of stations called visible light.

Here are the key things you need to know:

  • Speed of Light: In a vacuum (like outer space), all EM waves travel at the same incredible speed: 3.0 x 10⁸ m/s. This is the fastest speed possible in the universe!
  • Visible Light: The colours we see, from red to violet (think of a rainbow - ROYGBIV), make up the visible spectrum. Red light has the longest wavelength, and violet light has the shortest.
Did you know?

The reason you see lightning before you hear thunder is that light travels almost a million times faster than sound!

Key Takeaway

Light is a fast-travelling wave of energy. The light we see is a small part of the larger Electromagnetic Spectrum.


2. Reflection: Bouncing Light

Reflection is what happens when light hits a surface and bounces off. This is how mirrors work and why you can see your reflection in a calm lake.

Laws of Reflection

There are two simple rules that light always follows when it reflects off a smooth surface, like a plane mirror.

First, some key terms:

  • Incident Ray: The ray of light that hits the surface.
  • Reflected Ray: The ray of light that bounces off the surface.
  • Normal: An imaginary line drawn at 90° (perpendicular) to the surface at the point where the light hits. This is super important! We always measure angles from the normal.
  • Angle of Incidence (i): The angle between the incident ray and the normal.
  • Angle of Reflection (r): The angle between the reflected ray and the normal.

The Two Laws are:

  1. The incident ray, the reflected ray, and the normal all lie on the same plane. (This just means the light doesn't bounce off in a weird 3D direction).
  2. The angle of incidence is equal to the angle of reflection ($$i = r$$).
Common Mistake Alert!

Always measure your angles from the normal, NOT from the surface of the mirror. This is a very common trap in exam questions!

Images in a Plane Mirror

When you look in a flat (plane) mirror, you see an image that appears to be behind the mirror. This is a virtual image – you can't project it onto a screen because the light rays don't actually meet there; they only appear to come from that point.

Properties of an image in a plane mirror:

  • It's the same size as the object.
  • It's the same distance behind the mirror as the object is in front.
  • It's upright (not upside down).
  • It's laterally inverted (left and right are swapped). This is why the word "AMBULANCE" is written backwards on the front of the vehicle.
Key Takeaway

Reflection is light bouncing off a surface. The key rule is that the angle of incidence equals the angle of reflection ($$i=r$$), with angles measured from the normal. Plane mirrors form virtual, upright, same-sized images.


3. Refraction: Bending Light

Have you ever noticed that a straw in a glass of water looks bent? That's not magic, it's refraction! Refraction is the bending of light as it passes from one medium to another (like from air to water).

Why Does Light Bend?

Light bends because it changes speed when it enters a different material. Materials that slow light down more are called optically denser.

Analogy: Imagine you are pushing a lawnmower from a smooth pavement onto thick grass. As the right wheel hits the grass, it slows down, but the left wheel is still on the pavement moving fast. This causes the whole lawnmower to turn or 'bend'. Light does the same thing!

Rules for Bending:
  • When light enters an optically denser medium (e.g., air to glass), it slows down and bends towards the normal.
  • When light enters an optically less dense medium (e.g., glass to air), it speeds up and bends away from the normal.

Refractive Index and Snell's Law

How much does light bend? We measure this using the refractive index (n). It's a number that tells us how optically dense a material is. A higher `n` means a denser material.

The relationship between the angles and the refractive index is described by Snell's Law. For light going from a vacuum (or air, which is close enough) into a medium with refractive index `n`:

$$ n = \frac{\sin i}{\sin r} $$

Where:

  • n is the refractive index of the second medium.
  • i is the angle of incidence (in the first medium).
  • r is the angle of refraction (in the second medium).
Quick Review Box

Refraction Summary
What is it? The bending of light as it changes medium.
Why? Because the speed of light changes.
Rule: Less Dense -> More Dense = Bends TOWARDS normal.
Rule: More Dense -> Less Dense = Bends AWAY from normal.
Formula: $$ n = \frac{\sin i}{\sin r} $$


4. Total Internal Reflection (TIR)

This is a special and very useful case of refraction. It only happens when light is trying to go from a denser medium to a less dense medium (like from glass back into air).

As the angle of incidence (`i`) in the denser medium gets bigger, the angle of refraction (`r`) gets even bigger (remember, it bends away from the normal). Eventually, `r` will become 90°. The angle of incidence that causes this is called the critical angle (c).

If the angle of incidence is LARGER than the critical angle, the light doesn't refract at all. It reflects back into the denser medium as if it hit a perfect mirror. This is Total Internal Reflection.

Conditions for TIR:

  1. Light must be travelling from an optically denser medium to an optically less dense medium.
  2. The angle of incidence must be greater than the critical angle ($$i > c$$).

Memory Aid: Think "From Dense to Less, the Angle must be Best" (Denser to Less, Angle > critical).

Real-world Applications:
  • Optical Fibres: TIR is the secret behind super-fast broadband internet! Light signals bounce along the inside of thin glass or plastic fibres for hundreds of kilometres with almost no energy loss.
  • Sparkling Diamonds: A diamond's cut is designed to make light undergo TIR multiple times inside before it escapes, creating that brilliant sparkle.
Key Takeaway

TIR is a perfect reflection that happens when light hits the boundary of a less dense medium at an angle greater than the critical angle. It's the principle behind optical fibres.


5. Lenses: Focusing Light

Lenses are curved pieces of glass or plastic that use refraction to form images. They are in our eyes, glasses, cameras, and telescopes.

Two Types of Lenses:

  1. Converging Lens (Convex): Thicker in the middle. It brings parallel light rays together at a point called the principal focus or focal point (F).
  2. Diverging Lens (Concave): Thinner in the middle. It spreads parallel light rays out so they appear to come from a focal point (F).

Drawing Ray Diagrams

Don't worry, this isn't an art class! To find where an image is formed, we only need to draw two or three special rays. It's a step-by-step process.

Key Terms for Lenses:
  • Principal Axis: The line passing straight through the middle of the lens.
  • Optical Centre (O): The very centre of the lens.
  • Focal Length (f): The distance from the optical centre to the focal point (F).
The Three "Magic" Rays for a Converging Lens:
  1. A ray travelling parallel to the principal axis is refracted through the focal point (F) on the other side.
  2. A ray passing through the optical centre (O) goes straight through without bending.
  3. A ray passing through the focal point (F) on its way to the lens is refracted to travel parallel to the principal axis.

You only need to draw any two of these rays. The point where they cross is where the top of the image is formed!

The type of image formed by a converging lens depends on where the object is. It can be real (can be projected on a screen) or virtual (can't be projected, like a magnifying glass image).

The Thin Lens Formula

Drawing is great, but we can also calculate the image position using a formula. This is the Thin Lens Formula:

$$ \frac{1}{u} + \frac{1}{v} = \frac{1}{f} $$

Where:

  • u = object distance (distance from the object to the optical centre).
  • v = image distance (distance from the image to the optical centre).
  • f = focal length of the lens.
HKDSE Sign Convention: "REAL is Positive"

This is the most important part of using the formula. You need to know when to use positive (+) or negative (-) numbers.

THE GOLDEN RULE: If it's REAL, it's POSITIVE. If it's VIRTUAL, it's NEGATIVE.

Object distance (u):

  • Almost always a real object, so u is POSITIVE.

    • Focal length (f):
  • A converging (convex) lens has a real focal point, so f is POSITIVE.
  • A diverging (concave) lens has a virtual focal point, so f is NEGATIVE.

    • Image distance (v):
  • If the image is real (formed on the opposite side of the lens from the object), v is POSITIVE.
  • If the image is virtual (formed on the same side as the object), v is NEGATIVE.

    • Key Takeaway

    Lenses use refraction to form images. Converging lenses focus light and can form real or virtual images. Diverging lenses spread light and always form virtual images. Use the lens formula and the "REAL is Positive" rule to solve problems.


    6. The Wave Nature of Light

    So far, we've treated light as straight rays. This is called geometrical optics, and it's great for explaining mirrors and lenses. But light is also a wave, and sometimes it behaves like one. Two key wave behaviours are diffraction and interference.

    Diffraction: Spreading Out

    Diffraction is the spreading of waves as they pass through a narrow gap or around an obstacle. The effect is most significant when the size of the gap is similar to the wavelength of the wave.

    Imagine water waves in a harbour passing through a small opening in a wall. The waves don't just continue in a straight line; they spread out in a circular pattern on the other side. Light does this too!

    This is why you can't see infinitely sharp shadows. A tiny bit of light always bends around the edges.

    Interference: Waves Adding Up

    When two waves meet, they can add together or cancel each other out. This is called interference.

    • Constructive Interference: If the crests of two waves meet, they add up to make a bigger wave (brighter light).
    • Destructive Interference: If the crest of one wave meets the trough of another, they cancel each other out (darkness).

    This is what creates the shimmering rainbow colours you see on a soap bubble or an oil slick.

    Young's Double-Slit Experiment

    This is the classic experiment that proved light behaves like a wave. When light of a single colour (monochromatic light) is passed through two very narrow, close-together slits, it creates a pattern of bright and dark bands (fringes) on a screen.

    • The bright fringes are where constructive interference occurs.
    • The dark fringes are where destructive interference occurs.

    We can calculate the separation between the bright fringes using this formula:

    $$ \Delta y = \frac{\lambda D}{a} $$

    Where:

    • Δy = fringe separation (distance between two adjacent bright fringes).
    • λ = wavelength of the light.
    • D = distance from the slits to the screen.
    • a = distance between the centres of the two slits.

    The Diffraction Grating

    A diffraction grating is like a "super" double-slit. It's a slide with thousands of tiny, evenly spaced slits per centimetre. When light passes through it, it produces a much sharper and brighter interference pattern than a double-slit.

    The condition for a bright fringe (a maximum) is given by the grating equation:

    $$ d \sin \theta = n \lambda $$

    Where:

    • d = the distance between adjacent slits in the grating.
    • θ = the angle of the bright fringe from the centre.
    • n = the "order" of the fringe (n=0 is the central one, n=1 is the first bright fringe on either side, etc.).
    • λ = the wavelength of the light.
    Key Takeaway

    Diffraction (spreading) and interference (adding/cancelling) are strong evidence that light is a wave. Young's double-slit experiment and the diffraction grating demonstrate this by creating patterns of bright and dark fringes.