Light

The One Thing

Light generally travels in straight lines through a homogeneous transparent medium, and reflection changes its direction according to definite laws. The type and position of a reflecting surface, together with the object’s location, determine the nature, size, and position of the image formed.

Reactions, Processes and Experiments

Key Terms

• Light: A form of energy that produces the sensation of vision and enables us to see objects.
• Luminous object: An object that produces its own light, such as the Sun, a flame, or an electric bulb.
• Non-luminous object: An object that does not produce light but can be seen because it reflects light.
• Ray of light: An imaginary straight line showing the direction in which light travels.
• Beam of light: A group of light rays travelling together.
• Rectilinear propagation: The property by which light travels in a straight line in a homogeneous transparent medium.
• Transparent medium: A medium that allows most light to pass through it, such as clear glass.
• Opaque medium: A medium that does not allow light to pass through it, such as wood or metal.
• Translucent medium: A medium that allows only some light to pass through it, such as frosted glass.
• Reflection: The bouncing back of light into the same medium after it strikes a surface.
• Incident ray: The ray of light that falls on a reflecting surface.
• Reflected ray: The ray that travels away from the reflecting surface after reflection.
• Normal: An imaginary line drawn perpendicular to the reflecting surface at the point where the incident ray strikes.
• Angle of incidence: The angle between the incident ray and the normal.
• Angle of reflection: The angle between the reflected ray and the normal.
• Regular reflection: Reflection from a smooth surface in which parallel incident rays remain parallel after reflection.
• Diffused reflection: Reflection from a rough surface in which reflected rays scatter in different directions.
• Plane mirror: A mirror with a flat reflecting surface.
• Spherical mirror: A mirror whose reflecting surface forms part of a hollow sphere.
• Concave mirror: A spherical mirror whose reflecting surface curves inward, like the inside of a spoon.
• Convex mirror: A spherical mirror whose reflecting surface bulges outward, like the outside of a spoon.
• Pole: The geometrical 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 of a spherical mirror.
• Principal axis: The straight line passing through the pole and centre of curvature of a spherical mirror.
• Principal focus: The point on the principal axis where rays parallel to the axis meet or appear to come from after reflection.
• Focal length: The distance between the pole and the principal focus.
• Real image: An image formed by the actual meeting of light rays; it can be obtained on a screen.
• Virtual image: An image formed by the apparent meeting of rays; it cannot be obtained on a screen.
• Erect image: An image that has the same upright orientation as the object.
• Inverted image: An image that is upside down compared with the object.
• Magnification: The ratio of the height of the image to the height of the object.
• Lateral inversion: The apparent interchange of left and right in a mirror image: the left side of an object appears to be the right side, and vice versa.

Easily Confused

• Transparent, translucent and opaque media: Transparent media allow most light to pass through, translucent media allow only some light to pass through, and opaque media do not allow light to pass through.
• Regular and diffused reflection: Regular reflection from a smooth surface keeps parallel rays parallel and may form a clear image; diffused reflection from a rough surface scatters rays, although the laws of reflection still apply at every point.
• Incident ray and reflected ray: The incident ray approaches the reflecting surface; the reflected ray travels away from it.
• Angle of incidence and angle of reflection: Both angles are measured from the normal, not from the mirror surface; the two angles are equal.
• Real and virtual images: A real image is formed where reflected rays actually meet and can be obtained on a screen; a virtual image is formed where rays only appear to meet and cannot be obtained on a screen.
• Concave and convex mirrors: A concave mirror can form real or virtual images depending on object position; a convex mirror always forms a virtual, erect, and diminished image behind the mirror.
• Pole and centre of curvature: The pole is the geometrical centre of the reflecting surface; the centre of curvature is the centre of the sphere of which the mirror forms part.
• Radius of curvature and focal length: The radius of curvature is the distance between the pole and centre of curvature, while focal length is the distance between the pole and principal focus; for a spherical mirror, R = 2f.
• Luminous and non-luminous objects: A luminous object produces its own light; a non-luminous object is seen because it reflects light.
• Erect and inverted images: An erect image has the same upright orientation as the object; an inverted image is upside down compared with it.

What Gets Asked

• Explain or demonstrate rectilinear propagation of light: Questions may require the three-card experiment with aligned holes and a candle, or applications such as shadows, eclipses, and a pinhole camera. The mark-losing slip is failing to state that light travels in a straight line through a homogeneous transparent medium.
• State and apply the laws of reflection: The required statements are that the incident ray, reflected ray, and normal lie in the same plane, and that the angle of incidence equals the angle of reflection. A common error is measuring angles from the mirror surface rather than from the normal.
• Compare regular and diffused reflection: Regular reflection produces parallel reflected rays and may form a clear image; diffused reflection scatters reflected rays. The laws of reflection still apply at every point in diffused reflection.
• Describe plane-mirror images: The image is virtual, erect, laterally inverted, and the same size as the object; it is as far behind the mirror as the object is in front and cannot be formed on a screen. Omitting lateral inversion or the equal object–image distance costs marks.
• Determine the type of image formed by spherical mirrors: A concave mirror may form a real or virtual image depending on object position, whereas a convex mirror always forms a virtual, erect, and diminished image behind the mirror. Confusing these properties is a frequent error.
• Use mirror terminology, equations, and ray diagrams: Questions may involve R = 2f, m = image height / object height, the Cartesian sign convention, or standard rays through the focus and centre of curvature. The main slips are using incorrect signs or confusing the pole, centre of curvature, principal focus, and focal length.