Light – Reflection and Refraction Practice

If the question involves a mirror (concave or convex), use the mirror formula: 1/f = 1/v + 1/u. If it involves a lens (convex or concave), use the lens formula: 1/f = 1/v − 1/u. The key difference is the sign between the two terms on the right-hand side.

Negative. In the New Cartesian sign convention, the focus of a concave mirror lies in front of the mirror (in the direction opposite to incident light when light comes from the left), so f is negative for a concave mirror.

Magnification m = −v/u. For a real inverted image (real object, real image in front of the mirror), both v and u are negative, making m negative. For a virtual erect image (object between F and P of a concave mirror), v is positive and u is negative, making m positive.

A concave mirror converges reflected rays, so they can actually meet (real image) or appear to diverge (virtual image when object is between F and P). A convex mirror always diverges reflected rays — they never actually meet in front of the mirror, so the image is always virtual (appears to form behind the mirror).

In reflection, light bounces back into the same medium; the speed does not change. In refraction, light passes into a different medium and its speed changes, causing it to change direction. Reflection obeys the law ∠i = ∠r; refraction obeys Snell's law (sin i / sin r = constant).

Power is defined as P = 1/f where f must be in metres. The dioptre (D) is the unit of power: 1 D = 1 m⁻¹. Using focal length in centimetres would give an inconsistent unit; converting to metres gives the standard dioptre unit used in all optometry prescriptions.

Yes. When the object is placed beyond 2F₁ of a convex lens, the image forms between F₂ and 2F₂ and is diminished (smaller than object), real, and inverted. When the object is at infinity, the image is at F₂ — highly diminished. Only when the object is between F₁ and O does the convex lens give a magnified (virtual) image.

Hold the concave mirror facing a distant object (like a distant building or a bright window). Adjust the distance between the mirror and a screen (or a piece of white paper) until a sharp, clear, inverted image forms on the screen. This distance from the screen to the mirror is approximately equal to the focal length (since the distant object is approximately at infinity).

Both mean the image is twice the size of the object. m = +2 means the image is virtual, erect, and magnified (e.g., object between F and P of concave mirror). m = −2 means the image is real, inverted, and magnified (e.g., object between F and C of concave mirror).

Glass is optically denser than air, so light slows down when entering glass. This slowing causes the wavefront to tilt, bending the ray towards the normal. The degree of bending is quantified by Snell's law and the refractive index of glass.