Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Define and apply the laws of reflection and refraction.
• Use Snell's Law to calculate angles and refractive indices.
• Describe image formation by lenses using ray diagrams.
• Calculate the magnification produced by lenses.
• Define total internal reflection and critical angle.

Lesson summary

Geometrical optics is the study of light based on the assumption that light travels in straight lines (rays). This approach allows us to understand how light interacts with optical instruments like lenses and mirrors, forming images. It's a simplification of the wave nature of light, but a very useful one for understanding how we see, how cameras work, and many other everyday phenomena. For South African learners, understanding geometrical optics is crucial as it underpins technologies from cellphone cameras used for documenting community events to the design of solar panels that harness sunlight for energy.

Lesson notes

2.1 Reflection Definition: Reflection is the change in direction of a wave at a boundary between two different media so that the wave returns into the medium from which it originated.

Laws of Reflection: The angle of incidence is equal to the angle of reflection. This means that the angle between the incident ray and the normal (a line perpendicular to the surface at the point of incidence) is equal to the angle between the reflected ray and the normal.

Mathematically: θ i = θ r . The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane. This means that the reflection is predictable and occurs in a single, flat surface.

Types of Reflection: Specular Reflection: Occurs from smooth surfaces, producing a clear, reflected image (e.g., a mirror).

Diffuse Reflection: Occurs from rough surfaces, scattering the light in many directions, resulting in no clear image (e.g., a sheet of paper). 2.2 Refraction Definition: Refraction is the bending of a wave as it passes from one medium to another due to a change in the speed of the wave.

Snell's Law: Relates the angles of incidence and refraction to the refractive indices of the two media. The refractive index (n) of a medium is a measure of how much the speed of light is reduced in that medium compared to its speed in a vacuum.

Mathematically: n 1 sinθ 1 = n 2 sinθ 2 Where: n 1 is the refractive index of the first medium. θ 1 is the angle of incidence in the first medium. n 2 is the refractive index of the second medium. θ 2 is the angle of refraction in the second medium.

Explanation: Light bends towards the normal when it enters a medium with a higher refractive index (slower speed) and bends away from the normal when it enters a medium with a lower refractive index (faster speed). Air has a refractive index approximately equal to 1, while water is approximately 1.33, and glass is typically around 1.5.