Study Notes: Reflection of Light
Hey everyone! Ready to dive into the amazing world of light? In this chapter, we're going to explore reflection. It might sound like a big word, but it's something you see every single day. Reflection is the reason you can see your face in a mirror, see the moon in the sky, and even read this page! We'll break it down into simple parts, so let's get started on our journey to see how light works!
Bouncing Light: What is Reflection?
Imagine you're playing with a ball and you bounce it off a wall. Reflection is basically the same idea, but with light!
Reflection is what happens when light hits a surface and bounces off it. This is how we see most things around us.
There are two types of objects when we talk about light:
Luminous objects: These objects create their own light. Examples include the Sun, a light bulb, or a candle flame.
Non-luminous objects: These objects do not create their own light. We can only see them because they reflect light from a luminous source into our eyes. Examples include the Moon (it reflects sunlight), your desk, a book, and even your friends!
So, when you see a friend in a sunlit room, light from the Sun (or a lamp) is hitting your friend and bouncing off them into your eyes. That's reflection in action!
Key Takeaway
Reflection is light bouncing off a surface. It's how we see non-luminous objects.
The Rules of the Game: The Laws of Reflection
When light reflects off a smooth, flat surface like a mirror, it follows two very simple and predictable rules. Don't worry, these 'laws' are easier to remember than you think! To understand them, we first need to meet the key players.
Meet the Key Players
Let's imagine a ray of light hitting a flat mirror.
Plane Mirror: A perfectly flat and smooth mirror.
Incident Ray: The ray of light that travels towards the mirror and hits it. (Think: incident = coming in)
Reflected Ray: The ray of light that bounces off the mirror.
Normal: This is an imaginary line drawn at a perfect 90-degree angle (perpendicular) to the mirror's surface, right at the point where the incident ray hits.
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.
Quick Review: Avoid This Common Mistake!
A very common mistake is to measure the angles from the mirror surface. Always measure the angles of incidence and reflection from the Normal! The Normal is your starting line for measuring.
The Two Simple Laws
1. First Law of Reflection:
The incident ray, the reflected ray, and the normal all lie on the same flat surface, or plane.
(Easy way to think about it: You can draw all three lines perfectly on a single flat sheet of paper.)
2. Second Law of Reflection:
The angle of incidence is always equal to the angle of reflection.
We write this as a simple formula: $$i = r$$
(Easy way to think about it: The angle the light comes in at is the same angle it goes out at.)
Key Takeaway
Light reflection follows two rules: 1) The incident ray, reflected ray, and normal are all on the same plane. 2) The angle of incidence equals the angle of reflection (i = r).
Looking in the Mirror: Images in a Plane Mirror
When you look into a flat (plane) mirror, you see an image of yourself. But what kind of image is it? It has some very specific properties that scientists call its 'nature'.
What's Special About Your Reflection? (Nature of the Image)
An image formed by a plane mirror is always:
Upright: The image is the right way up, not upside down.
Same Size: The image is the exact same size as the object. The mirror doesn't make you look bigger or smaller.
Same distance behind the mirror: If you stand 2 metres in front of a mirror, your image appears to be 2 metres behind the mirror.
Virtual: This means the image isn't really located behind the mirror. The light rays only appear to come from that point. You can't project a virtual image onto a screen. (It's like a 'ghost' image that you can see, but can't touch!)
Laterally Inverted: This is a fancy way of saying that left and right are swapped. If you raise your right hand, your reflection in the mirror appears to raise its left hand.
Drawing the Secret: How to Draw a Ray Diagram
We can prove where the image is formed by drawing a simple diagram. Don't worry if this seems tricky at first, just follow the steps!
Step 1: Draw the plane mirror (as a straight line with dashes on one side), the object (like an arrow), and a point for the eye.
Step 2: Draw at least two incident rays from the top of the object to the mirror.
Step 3: At each point where a ray hits the mirror, draw the normal (a dotted line at 90° to the mirror).
Step 4: For each incident ray, draw the reflected ray. Make sure the angle of reflection equals the angle of incidence (r = i). These reflected rays should travel towards the eye.
Step 5: The reflected rays will be spreading apart. Use a ruler and a dotted line to extend them backwards behind the mirror.
Step 6: The point where these dotted lines meet is the location of the top of the virtual image. You can now draw the image (usually with a dotted line to show it's virtual).
Key Takeaway
An image in a plane mirror is Upright, Same Size, Same Distance behind, Virtual, and Laterally Inverted. A helpful mnemonic could be: Uncle Sam's Son Visits London.
Reflection All Around Us: Real-World Applications
Reflection isn't just for looking at yourself! It has many clever and important uses.
Periscopes
A simple periscope, like the kind used in submarines to see above the water, uses two plane mirrors. The mirrors are set parallel to each other at a 45-degree angle. Light from an object above bounces off the top mirror, down the tube, and then bounces off the bottom mirror into the viewer's eye. This allows you to see over obstacles!
Kaleidoscopes
This fun toy uses reflection to create beautiful patterns. Inside a kaleidoscope, there are typically three long plane mirrors arranged in a triangle. Coloured beads at the end are reflected in the mirrors over and over again, creating a stunning, symmetrical pattern that changes as you turn it.
Did you know?
The word "AMBULANCE" is often written backwards on the front of the vehicle. Why? It's because of lateral inversion! When a driver in front looks in their rear-view mirror, the word will be flipped the right way around, allowing them to read it quickly and move out of the way. Cool, right?
Everyday Mirrors
Of course, the most common application is the simple mirror we use every day in bathrooms, bedrooms, and in cars (rear-view mirrors) to see what is behind us. They help us stay safe and check our appearance.
Key Takeaway
Reflection is used in clever devices like periscopes and kaleidoscopes, and has important safety uses, like on the front of an ambulance.