Course: Separate Physics only | Tier: Foundation + Higher | Linked equation: magnification = image height ÷ object height
Notes
Lenses
- A lens forms an image by refracting light.
- Convex (converging) lens: parallel rays of light are brought together at the principal focus. It can form a real or a virtual image – a virtual, magnified image when the object is closer than the principal focus (a magnifying glass).
- Concave (diverging) lens: parallel rays spread out as if they came from the principal focus. It always forms a virtual image.
- The distance from the lens to the principal focus is the focal length.
- A real image can be projected onto a screen; a virtual image cannot.
- Magnification = image height ÷ object height. It has no units (both heights must be in the same unit).
Visible light and colour
- Each colour has a narrow band of wavelengths and frequencies (red has the longest wavelength, violet the shortest).
- Specular reflection: from a smooth surface, in a single direction. Diffuse reflection: from a rough surface, scattered in many directions.
- Colour filters transmit certain wavelengths and absorb all the others.
- An opaque object’s colour depends on which wavelengths it reflects most. White objects reflect all wavelengths equally; black objects absorb all wavelengths.
- Transparent and translucent objects transmit light.
Questions
Q1 (F) What does a convex lens do to parallel rays of light?
Show answer
It refracts them so they converge (meet) at the principal focus.
Q2 (F) What is the difference between a real image and a virtual image?
Show answer
A real image is formed where rays actually meet and can be shown on a screen. A virtual image is where rays only appear to come from, so it can’t be shown on a screen.
Q3 (F) An object 2.0 cm tall forms an image 6.0 cm tall. Calculate the magnification.
Show answer
Magnification = 6.0 ÷ 2.0 = 3 (no units)
Q4 (F) Why does a red apple look red in white light?
Show answer
It reflects red wavelengths and absorbs the other colours.
Q5 (F/H) What is the difference between specular and diffuse reflection?
Show answer
Specular reflection is from a smooth surface, so light is reflected in a single direction (a clear image, like a mirror). Diffuse reflection is from a rough surface, so light is scattered in many directions.
Q6 (F/H) White light shines through a green filter onto a red object. What colour does the object appear, and why?
Show answer
Black. The filter only lets green light through. The red object absorbs green light and there is no red light for it to reflect.
Q7 (F/H) What type of image does a concave lens always produce?
Show answer
A virtual image (upright and smaller than the object).
Q8 (H) A magnifying glass produces an image 45 mm tall with a magnification of 1.5. How tall is the object?
Show answer
F: magnification = image height ÷ object height
I: 1.5 = 45 ÷ object height
F: object height = 45 ÷ 1.5
A: 30 mm
