Optics — A-Level Physics Revision

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What is Optics?

Optics is the branch of physics that studies the behaviour and properties of light. In A-Level Physics, this topic focuses on refraction, exploring how light bends when it passes from one medium to another, governed by Snell's Law and the concept of refractive index. You will also learn to construct ray diagrams for converging and diverging lenses to determine the nature, position, and magnification of an image.

Board notes: This topic is a core component of all A-Level Physics specifications (AQA, Edexcel, OCR). The complexity of lens combinations and the required precision in ray diagrams can vary. OCR places a particular emphasis on the applications of optics, such as in telescopes and medical imaging.

Worked example

Common mistakes

• 1Mixing up the angles in Snell's Law. Students often confuse the angle of incidence and the angle of refraction, or the refractive indices of the two media. Remember that n1 sin(θ1) = n2 sin(θ2), where the '1's and '2's correspond to the same medium.
• 2Incorrectly drawing principal rays for lens diagrams. A common error is to not draw the ray passing through the optical centre as a straight, undeviated line, or to incorrectly refract the ray parallel to the principal axis through the wrong focal point.
• 3Confusing real and virtual images. Real images are formed where light rays actually converge and can be projected onto a screen, whereas virtual images are formed where rays only appear to diverge from and cannot be projected.

Optics exam questions

Exam-style questions for Optics with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• What is the critical angle in optics?

  The critical angle is the angle of incidence beyond which total internal reflection occurs for light travelling from a denser medium to a less dense medium. It is the angle of incidence for which the angle of refraction is 90°.

• How do you calculate the power of a lens?

  The power of a lens (in dioptres, D) is the reciprocal of its focal length in metres (P = 1/f). Converging lenses have positive power, while diverging lenses have negative power.