Introduction to Wave Interaction
Welcome to the study of Wave Interaction! Have you ever wondered why your reflection appears in a mirror, why a straw looks "broken" in a glass of water, or why an apple looks red? These are all examples of how waves interact with different materials. In this section, we will explore how light and other electromagnetic waves behave when they hit a surface. Don't worry if it sounds complicated—we'll break it down into simple steps!
1. What Happens When a Wave Meets a Material?
When an electromagnetic wave (like light) hits a substance, four main things can happen. Most materials do a mix of these depending on the wavelength of the wave and the type of material.
- Reflection: The wave "bounces" off the surface.
- Absorption: The energy of the wave is taken in by the material (often turning into heat).
- Transmission: The wave passes straight through the material.
- Refraction: The wave passes through but changes speed and direction.
The Speed Connection
Waves change direction (refract) because they change velocity when they enter a different material. For example, light travels slower in glass than it does in air.
Analogy: Imagine a shopping trolley moving from a smooth pavement onto a patch of grass at an angle. As the first wheel hits the grass, it slows down, causing the whole trolley to pivot and change direction. This is exactly how light behaves!
Quick Review: The ATRR Rule
To remember what happens to waves, just think ATRR:
Absorb, Transmit, Reflect, Refract.
Key Takeaway: Whether a wave is reflected, absorbed, or transmitted depends on the material it hits and the wavelength of the wave itself.
2. Lenses and Vision
Lenses use refraction to bend light and create images. There are two main types you need to know for your OCR GCSE:
Convex Lenses (Converging)
A convex lens is thicker in the middle than at the edges. It brings light rays together to a single point.
Memory Aid: Think "Convex is Fat in the Middle."
- Use: Used to correct long-sightedness (where you can't see things close up).
- How it works: It helps the eye bend light more so the image forms on the retina rather than behind it.
Concave Lenses (Diverging)
A concave lens is thinner in the middle and curves inwards. It spreads light rays out.
Memory Aid: A concave lens is like the entrance to a cave—it goes inwards!
- Use: Used to correct short-sightedness (where you can't see things far away).
- How it works: It spreads the light out before it hits your eye so the image doesn't form too early.
Did you know? Your eye actually contains a natural convex lens that changes shape to help you focus on objects at different distances!
Key Takeaway: Convex lenses bring light together (converge); concave lenses spread light out (diverge).
3. Drawing Ray Diagrams
Ray diagrams are simple drawings that show the path light takes. When drawing these, always use a sharp pencil and a ruler!
The Normal Line
Before drawing any rays, you must draw the Normal. This is an imaginary dotted line at 90 degrees (right angles) to the surface where the light hits. We measure all our angles from this line.
Rules for Reflection
The Law of Reflection is very simple:
Angle of Incidence = Angle of Reflection
In symbols: \( i = r \)
Rules for Refraction
When light goes from a less dense material (like air) into a more dense material (like glass):
- The light slows down.
- The light bends towards the normal.
Common Mistake: Students often forget to measure the angle from the Normal line. Always start your protractor at the dotted line, not the surface of the glass!
Key Takeaway: Always draw a Normal line at 90° to the surface. Angle in equals angle out for reflection!
4. The Physics of Colour
Why is a red apple red? It’s all about differential absorption and reflection. White light is actually made of all the colors of the rainbow (Red, Orange, Yellow, Green, Blue, Indigo, Violet).
How we see Coloured Objects
- An object's colour is determined by which wavelengths of light it reflects.
- A red apple absorbs every colour except red. It reflects the red light back to your eyes.
- A white object reflects all wavelengths.
- A black object absorbs all wavelengths (which is why black clothes feel hotter in the sun!).
Coloured Filters
Filters work by transmission. A blue filter only transmits (lets through) blue light. It absorbs all the other colours. If you look at a red apple through a blue filter, the apple will look black because there is no red light being let through the filter!
Specular Reflection vs. Scattering
- Specular Reflection: This happens on smooth surfaces (like a mirror). All light rays reflect in the same direction, giving a clear reflection.
- Scattering (Diffuse Reflection): This happens on rough surfaces (like paper). The light rays hit the bumps and reflect in many different directions. This is why you can see the paper, but you can't see your face in it!
Key Takeaway: Objects appear to be the colour of the light they reflect. Filters only let through the colour they are.
Quick Review Box
Refraction: Bending of light due to a change in speed.
Convex Lens: () - Converges light, fixes long-sight.
Concave Lens: )( - Diverges light, fixes short-sight.
Reflection: Angle \( i \) = Angle \( r \).
Colour: Objects reflect their own colour and absorb the rest.