Light - Reflection and Refraction

The One Thing

Reflection is the return of light into the same medium, whereas refraction is the change in direction of light as it passes between transparent media. Mirrors and lenses use these effects to form images whose position, size, and nature can be predicted using ray diagrams, formulas, and magnification.

Reactions, Processes and Experiments

Key Terms

• Light ray: A straight-line path showing the direction in which light travels.
• Reflection: The return of light into the same medium after it strikes a surface.
• Incident ray: The ray of light that falls on a reflecting or refracting surface.
• Reflected ray: The ray that bounces back from a reflecting surface.
• Normal: A line drawn perpendicular to the surface at the point where light strikes it.
• Laws of reflection: The angle of incidence equals the angle of reflection, and the incident ray, reflected ray, and normal lie in the same plane.
• Spherical mirror: A mirror whose reflecting surface forms part of a sphere.
• Concave mirror: A converging mirror with its reflecting surface curved inward.
• Convex mirror: A diverging mirror with its reflecting surface curved outward.
• Pole: The centre of the reflecting surface of a spherical mirror.
• Centre of curvature: The centre of the sphere of which a spherical mirror is a part.
• Radius of curvature: The distance between the pole and the centre of curvature.
• Principal axis: The straight line passing through the pole and centre of curvature of a spherical mirror.
• Principal focus of a mirror: The point on the principal axis where parallel rays meet or appear to come from after reflection.
• Focal length: The distance between the pole and principal focus of a mirror, or between the optical centre and focus of a lens.
• Refraction: The change in direction of light when it travels obliquely from one transparent medium to another.
• Refractive index: A measure of how much a medium slows and bends light, given by n = c/v, where c is the speed of light in vacuum and v is its speed in the medium.
• Laws of refraction: The incident ray, refracted ray, and normal lie in the same plane, and for a given pair of media sin i/sin r is constant.
• Convex lens: A converging lens that is thicker at the centre and bends parallel rays towards the principal focus.
• Concave lens: A diverging lens that is thinner at the centre and makes parallel rays appear to come from the principal focus.
• Optical centre: The central point of a thin lens through which a ray passes without significant deviation.
• Real image: An image formed by the actual meeting of light rays; it can generally be obtained on a screen.
• Virtual image: An image formed by the apparent meeting of light rays; it cannot be obtained on a screen.
• Magnification: The ratio of image size to object size, showing whether the image is enlarged or diminished.

Easily Confused

• Reflection and refraction: Reflection returns light into the same medium, whereas refraction changes the direction of light as it enters another medium.
• Concave and convex mirrors: A concave mirror is converging and may form real or virtual images; a convex mirror is diverging and always forms a virtual, erect, and diminished image.
• Convex and concave lenses: A convex lens is converging and may form real or virtual images; a concave lens is diverging and always forms a virtual, erect, and diminished image.
• Real and virtual images: A real image is formed by the actual meeting of rays and can generally be obtained on a screen; a virtual image is formed by the apparent meeting of rays and cannot be obtained on a screen.
• Mirror and lens magnification: For a mirror, m = h_i/h_o = -v/u; for a lens, m = h_i/h_o = v/u.
• Mirror and lens sign conventions: Distances are measured from the pole of a mirror or the optical centre of a lens; distances in the direction of incident light are positive, while those opposite to it are negative.
• Focal length and radius of curvature: For a spherical mirror, focal length is related to radius of curvature by f = R/2; radius of curvature is the distance between the pole and centre of curvature.
• Converging and diverging devices: A concave mirror and convex lens converge parallel rays, whereas a convex mirror and concave lens cause rays to diverge or appear to diverge.

What Gets Asked

• Definitions and distinctions: Questions may ask for the meanings of reflection, refraction, incident ray, reflected ray, normal, real image, virtual image, or refractive index. Marks are lost by treating reflection as bending between media or refraction as return into the same medium.
• Laws and ray diagrams: Questions may require the laws of reflection or refraction and standard rays. Marks are lost by omitting the common-plane condition or by drawing the ray parallel to a concave mirror or convex lens incorrectly.
• Image characteristics: Questions may ask whether an image is real or virtual, erect or inverted, and enlarged or diminished. Marks are lost by failing to distinguish the always virtual, erect, and diminished images of a convex mirror and concave lens from the variable images of a concave mirror and convex lens.
• Numerical formula questions: Questions may require f = R/2, 1/f = 1/v + 1/u, 1/f = 1/v - 1/u, m = h_i/h_o = -v/u, or m = h_i/h_o = v/u. Marks are lost by using the mirror magnification equation for a lens or the lens formula for a mirror.
• Refraction and refractive index: Questions may ask for n = c/v or the direction of bending between rarer and denser media. Marks are lost by reversing “towards the normal” and “away from the normal.”
• Lens power and sign convention: Questions may ask for P = 1/f and the sign of power. Marks are lost by failing to convert focal length to metres or by assigning positive power to a concave lens.