Physics Unit P2: Light Study Notes
Welcome to the study notes for Light! Light is all around us, enabling us to see the world, creating colourful rainbows, and making devices like cameras, glasses, and telescopes work. Don't worry if physics ray diagrams seem a bit daunting at first—we will break down every concept step-by-step with clear rules, simple analogies, and key exam tips.
---1. Nature, Emission, and Travel of Light
Luminous vs Non-Luminous Objects
Everything we see is because light enters our eyes. However, objects produce and interact with light in two distinct ways:
• Luminous Objects: Objects that create and give off their own light. Examples include the Sun, light bulbs, lit candles, and glowing stars.
• Non-Luminous (Illuminated) Objects: Objects that do not make their own light. We only see them because light from a luminous source hits them and bounces (reflects) into our eyes. Examples include the Moon, a book, a table, and your clothes.
How Light Travels (Rectilinear Propagation)
Light travels in straight lines through uniform substances (media) such as air, glass, or empty space. The scientific term for travelling in straight lines is rectilinear propagation.
Shadow Formation: Because light travels in straight lines and cannot bend around solid opaque objects, shadows are formed when an object blocks the light rays.
Key Takeaway: Luminous objects emit light; non-luminous objects reflect light. Light always travels in straight lines in a uniform medium.
---2. The Reflection of Light
Reflection occurs when light rays hit a surface and bounce off it.
Key Terms and the Law of Reflection
To understand reflection, physicists draw a line called the normal:
• The Normal: An imaginary reference line drawn perpendicular (\(90^\circ\)) to the reflective surface at the exact point where the light ray strikes.
• Incident Ray: The incoming light ray travelling towards the mirror.
• Reflected Ray: The light ray that bounces off the mirror.
• 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 Law of Reflection:
$$\text{Angle of incidence } (i) = \text{Angle of reflection } (r)$$
$$\(i = r\)$$
Drawing Ray Diagrams for Reflection
When drawing ray diagrams in exams, you must follow strict scientific rules:
1. Always use a sharp pencil and a ruler to draw straight lines.
2. Always draw an arrowhead on the ray to indicate the direction the light is travelling.
3. Always draw the normal as a dashed line at \(90^\circ\) to the surface.
Common Examiner Trap: Always measure angles from the normal, never from the mirror surface itself! If the angle between the ray and the mirror is \(30^\circ\), the angle of incidence is \(90^\circ - 30^\circ = 60^\circ\).
Images in a Plane (Flat) Mirror
When you look at yourself in a flat mirror, the image produced has four key characteristics:
1. Virtual: The image cannot be formed or projected onto a screen. The light rays only appear to come from behind the mirror.
2. Upright: The image is the right way up (not upside down).
3. Same Size: The image is exactly the same size as the object.
4. Laterally Inverted: The image is flipped sideways (your left hand appears as the image's right hand).
5. Equal Distance: The perpendicular distance from the object to the mirror is equal to the perpendicular distance from the image to the mirror.
Key Takeaway: For all reflection, \(i = r\). Both angles are measured to the normal. Plane mirror images are virtual, upright, laterally inverted, and the same distance behind the mirror as the object is in front.
---3. Refraction of Light
What is Refraction?
Refraction is the change in direction of a light ray caused by a change in speed when it passes from one optical medium into another of different optical density (e.g., from air into glass or water).
Everyday Analogy: Imagine pushing a shopping trolley from a smooth concrete floor onto a patch of thick mud at an angle. The wheel that hits the mud first slows down before the other wheel, causing the entire trolley to pivot and change direction.
The Rules of Bending
• From Less Dense to More Dense (e.g., Air into Glass or Water): Light slows down and bends towards the normal. In this case, the angle of incidence is greater than the angle of refraction (\(i > r\)).
• From More Dense to Less Dense (e.g., Glass or Water into Air): Light speeds up and bends away from the normal. In this case, the angle of refraction is greater than the angle of incidence (\(r > i\)).
• Normal Incidence (\(i = 0^\circ\)): If a light ray enters a medium along the normal at an angle of \(90^\circ\) to the surface (\(i = 0^\circ\)), its speed changes, but its direction does not change (it passes straight through).
Refraction Through a Rectangular Glass Block
When light passes through a parallel-sided rectangular glass block:
1. At the first surface (air to glass), it slows down and bends towards the normal.
2. Inside the block, it travels in a straight line.
3. At the second surface (glass to air), it speeds up and bends away from the normal by the exact same amount.
4. The emergent ray leaves the block parallel to the original incident ray, but shifted sideways (laterally displaced).
Key Takeaway: Light bends towards the normal when slowing down (entering a denser medium) and bends away from the normal when speeding up (entering a less dense medium).
---4. Dispersion of White Light
What is Dispersion?
Dispersion is the splitting of white light into its component spectrum of colours when passed through a triangular glass prism.
The Visible Spectrum
White light is a mixture of all the colours of the visible spectrum. In order from longest wavelength to shortest wavelength:
• Red
• Orange
• Yellow
• Green
• Blue
• Indigo
• Violet
Memory Trick: Remember the name ROY G. BIV or the phrase "Richard Of York Gave Battle In Vain".
Why Does Dispersion Happen?
In a vacuum or in air, all colours of light travel at the same speed. However, when entering glass:
• Different colours have different wavelengths and travel at different speeds in glass.
• Red light has the longest wavelength, is slowed down the least, and therefore bends (refracts/deviates) the least.
• Violet light has the shortest wavelength, is slowed down the most, and therefore bends (refracts/deviates) the most.
Because each colour refracts by a slightly different amount at both faces of the triangular prism, they spread out into a continuous band of colours.
Key Takeaway: Dispersion occurs because different colours travel at different speeds in glass. Red deviates the least; violet deviates the most.
---5. Lenses and Ray Diagrams
Lenses use refraction to change the direction of light rays and form images.
Types of Lenses
• Converging (Convex) Lens: Thicker in the middle than at the edges. It brings parallel light rays together towards a single point (converges them).
• Diverging (Concave) Lens: Thinner in the middle than at the edges. It spreads parallel light rays outwards (diverges them).
Key Optical Terms for Lenses
• Principal Axis: An imaginary horizontal line passing straight through the optical centre of the lens, perpendicular to the plane of the lens.
• Principal Focus (\(F\)): The point on the principal axis where rays travelling parallel to the principal axis meet (converge) after passing through a convex lens (or appear to spread out from in a concave lens).
• Focal Length (\(f\)): The distance from the optical centre of the lens to the principal focus (\(F\)).
Describing Images Formed by Lenses
Whenever you are asked in an exam to describe an image formed by a lens, you must state three properties:
1. Real or Virtual:
- Real image: Light rays actually meet and pass through the image point; it can be projected onto a screen.
- Virtual image: Light rays only appear to come from behind the lens; it cannot be projected onto a screen.
2. Inverted or Upright:
- Inverted: Upside down compared to the object.
- Upright: The same way up as the object.
3. Magnified, Diminished, or Same Size:
- Magnified: Larger than the original object.
- Diminished: Smaller than the original object.
- Same size: Exactly the same height as the original object.
Key Takeaway: Convex lenses converge light to a principal focus (\(F\)). Images are always described using three words: real/virtual, upright/inverted, and magnified/diminished/same size.
---6. Summary & Exam Success Checklist
Before sitting your Unit P2 exam, double-check that you can:
• State the difference between luminous and non-luminous objects.
• State the law of reflection (\(i = r\)) and draw accurate ray diagrams using a ruler and arrows.
• Recall the properties of an image in a plane mirror (virtual, upright, laterally inverted, same size, equal distance).
• Explain refraction in terms of changing speed at a boundary.
• State which way light bends when entering a denser medium (towards the normal) and a less dense medium (away from the normal).
• List the colours of the visible spectrum in order (ROYGBIV) and explain that red bends least and violet bends most during dispersion.
• Identify converging (convex) and diverging (concave) lenses.
• Define principal axis, principal focus (\(F\)), and focal length (\(f\)).
• Fully describe any image using the three essential descriptors (real/virtual, inverted/upright, magnified/diminished).