Optics

What the exam asks

Expect a ray diagram, a boundary with two media labelled, or a lens with an object at a marked distance. Snell's law and the lens equation are given to you. The commonest items are: decide which way a ray bends at a boundary, decide whether total internal reflection happens, and classify an image as real or virtual, upright or inverted, magnified or diminished. The trap that costs the most is reading an angle from the surface instead of from the normal, which silently turns theta into 90 minus theta and produces a wrong answer that looks entirely plausible and sits in the option list waiting. The second is claiming total internal reflection for light heading into the denser medium, where it cannot happen at any angle at all. The third is thinking that covering half a lens removes half the image: it removes half the light.

Optics is what happens to light at a boundary, and what a lens or mirror does with it afterwards. Snell's law, the lens equation and the magnification ratio are all printed in the stem, along with the sign convention if the item needs one. Use the convention you are given rather than the one you memorised, because courses differ on it and the stem is the authority.

What you carry in is the qualitative behaviour. Light entering a slower medium bends toward the normal; leaving into a faster one it bends away, and past a certain angle it does not leave at all. A converging lens makes a real inverted image of anything beyond its focal point, and a virtual upright one of anything inside it. A diverging lens and a convex mirror make virtual, upright, diminished images of everything, from every position, always.

The exam likes ray diagrams and it likes the real-versus-virtual distinction, because that distinction separates a candidate who has a picture in their head from one who has only a formula.

What to hold

• Every refraction angle is measured from the normal, the line perpendicular to the surface at the point of incidence, and never from the surface itself.
• Light slows on entering a medium of higher refractive index and bends toward the normal. Going the other way it speeds up and bends away from the normal.
• Refraction changes a wave's speed and its wavelength and leaves its frequency alone, because the frequency is set by the source.
• n = c/v, so a larger refractive index means slower light in that medium.
• Total internal reflection needs two conditions together: the light must be going from the denser medium toward the less dense one, and the angle of incidence must be above the critical angle. Either one alone is not enough.
• At exactly the critical angle the refracted ray is bent to 90 degrees from the normal and grazes along the boundary. Above it, no light gets out at all.
• sin(critical angle) = n2/n1, so a bigger index gives a smaller critical angle. Glass to air is about 42 degrees, water to air about 49 degrees.
• A real image is a place where rays actually meet, so a screen put there shows it. A virtual image is where rays only appear to have come from, and no screen will ever show it.
• For a single lens or mirror, real images are inverted and virtual images are upright. The two travel together.
• A converging lens gives a real inverted image of an object beyond its focal point, and a virtual upright magnified image of an object inside it, which is what a magnifying glass is.
• A diverging lens and a convex mirror give a virtual, upright, diminished image wherever the object is put. No object position changes this.
• Magnification is image size over object size, and it equals image distance over object distance. A magnification above one means bigger, and says nothing about whether the image is real or virtual.