Cambridge O Level Physics · Syllabus 5054 · Waves
Reflection
What is Reflection?
Reflection is the change in direction of a wave when it bounces back from a boundary or plane surface; the wavelength, frequency and speed are unchanged.
This definition is part of the Waves chapter in Cambridge O Level Physics.
Reflection in context
When light meets a plane mirror it is reflected so that the angle of incidence equals the angle of reflection, both measured between the ray and the normal — the line drawn at 90° to the surface at the point where the ray strikes it. The image in a plane mirror is virtual, the same size as the object and the same perpendicular distance behind the mirror as the object is in front. It is found by extending the reflected rays backwards with dashed construction lines until they appear to meet.
Common mistakes with Reflection
- 7. “Angles in optics are measured from the surface.” Why wrongEvery optical law in this topic — \(i = r\), \(n = \sin i / \sin r\), \(n = 1/\sin c\) — is defined with respect to the normal. Measuring from the surface gives the complement, \(90^\circ - i\). CorrectDraw the dashed normal at 90° to the surface at the point of incidence first, then measure every angle from it. Say this“The angle of incidence is the angle between the incident ray and the normal.” CheckA ray strikes a mirror at 25° to the surface. State the angle of reflection. (65°.)
- 8. “A plane-mirror image lies on the mirror.” Why wrongTo photograph your reflection sharply, a camera must focus on a point twice your distance from the glass, not on the glass surface. CorrectThe image is the same perpendicular distance behind the mirror as the object is in front, on the line perpendicular to the mirror through the object. Say this“The image is formed the same perpendicular distance behind the mirror as the object is in front of it.” CheckA person stands 1.2 m from a mirror. How far is the person from the image? (2.4 m.)
- 12. “Total internal reflection occurs at the critical angle.” Why wrongAt exactly \(c\) a refracted ray still exists — it travels along the boundary at 90° to the normal. CorrectTotal internal reflection needs the angle of incidence to be strictly greater than \(c\). Say this“Total internal reflection occurs when the angle of incidence is greater than the critical angle.” CheckWater has \(c = 49^\circ\). A ray inside the water strikes the surface at 49°. What happens?
Questions students ask about Reflection
Why are angles in reflection and refraction always measured from the normal, not the surface?
Every optical law in this chapter — the law of reflection, \(n = \sin i/\sin r\), and \(n = 1/\sin c\) — is defined with respect to the normal, the line drawn at 90° to the surface at the point where the ray strikes it. Measuring from the surface instead gives the complement of the angle, \(90^\circ - i\), which is not what the law describes. Always draw the dashed normal first.