Optics

Using the lens formula 1/f = 1/v - 1/u, we substitute f = 20 cm and v = 30 cm for a real image to get 1/20 = 1/30 - 1/u, which solves to u = -60 cm. Option A is incorrect because a 10 cm object distance is less than the focal length, which would produce a virtual image on the same side rather than a real image at 30 cm.

The critical angle for total internal reflection is determined by the ratio of the refractive indices of the two interfacing media, as described by Snell's law. Option A is incomplete because focusing only on wavelength ignores how both media must interact to determine the boundary's refractive properties.

A plane mirror forms a virtual and erect image because the reflected light rays diverge and only appear to meet behind the mirror. A diagram for this topic isn't available yet — the worked example below walks the same reasoning step by step.

• !Option A is incorrect because a 10 cm object distance is less than the focal length, which would produce a virtual image on the same side rather than a real image at 30 cm.

Answer each from memory — you'll see instantly whether you're right and why.

Run a focused 10-question mini-mock on Optics and see it stick.