Refraction, lenses and optical evidence

## Big idea: refraction, lenses and optical evidence - **Key concept: Systems.** Light changes speed when it crosses into another medium. At oblique incidence this changes direction; lenses use refraction to form images. - **Related concepts: Models and evidence.** Use a scientific explanation to make predictions, then test it against observations. - **Global context: Scientific and technical innovation.** Lenses are used in cameras and instruments to form useful images. ## Reflection of Light - A **normal** is drawn at right angles to the boundary between two media. - Angles are measured between the ray and the normal: **angle of incidence (i)** and **angle of reflection (r)**. - **Law of reflection**: angle of incidence = angle of reflection (i = r). - In a plane mirror, the image is **virtual**, **same size**, **same distance behind** the mirror as the object is in front, and **laterally inverted**. - Ray diagrams show incident and reflected rays with arrows indicating direction. ## Investigating Reflection - **Aim**: To investigate reflection by a plane mirror. - **Independent variable**: angle of incidence; **dependent variable**: angle of reflection. - Use a ray box, protractor, paper, pencil, ruler, and plane mirror. - Mark incident and reflected ray positions, then measure angles with a protractor. - Repeat for different angles of incidence to verify i = r. ## Refraction of Light - Refraction occurs when light passes between two transparent media of different **optical density**. - From less dense to more dense (e.g., air to glass): light bends **towards** the normal. - From more dense to less dense (e.g., glass to air): light bends **away** from the normal. - If light travels along the normal, it does **not bend**. - Frequency remains constant; speed and wavelength change. ## Refractive Index - Refractive index n = speed of light in vacuum / speed of light in material. - Also n = sin i / sin r (Snell's law). - Refractive index has no unit. For the ordinary transparent materials studied here, such as glass and water, it is greater than 1 relative to a vacuum. - Higher n means more optically dense (e.g., diamond n ≈ 2.4, glass n ≈ 1.5). ## Total Internal Reflection - **Total internal reflection (TIR)** occurs when light travels from a denser to a less dense medium and the angle of incidence exceeds the **critical angle (c)**. - Critical angle: angle of incidence for which angle of refraction = 90°. - Relationship: sin c = 1/n. - Higher n gives smaller critical angle. - Uses: optical fibres (communications, endoscopes), prisms in periscopes, binoculars. ## Ray Diagrams for Lenses - **Converging (convex) lens**: brings parallel rays to a focus at the **principal focus (f)**. - **Diverging (concave) lens**: makes parallel rays diverge; virtual focus on same side as incoming light. - **Focal length**: distance from lens centre to principal focus. - Three key rays: through optical centre (undeviated), parallel to axis (through focus), through focus (emerges parallel). ## Real & Virtual Images - **Real image**: formed where rays converge and can be projected on a screen. A single converging lens forms an inverted real image of a real object outside its focal length. - **Virtual image**: rays appear to diverge from its position; it cannot be projected directly on a screen. The single-lens examples studied here produce upright virtual images of real objects. - Converging lens can produce real or virtual images depending on object position. - Diverging lens always produces a **virtual, upright, diminished** image. ## Correcting Sight & Dispersion - **Long-sightedness**: image forms behind retina; corrected with **converging (convex) lens**. - **Short-sightedness**: image forms in front of retina; corrected with **diverging (concave) lens**. - **Dispersion**: white light splits into colours when refracted by a prism; violet bends most, red least. - Visible spectrum: red, orange, yellow, green, blue, indigo, violet (longest to shortest wavelength). - **Monochromatic light**: single frequency/colour (e.g., laser). ## Think like a scientist - Use a school ray box to compare angles of incidence and refraction as light enters a glass block; never direct it towards eyes. - **Comparison:** the angle of incidence. **Outcome:** the angle of refraction measured from the normal. - **Control:** use the same glass block and light source. Explain why this makes the comparison fairer. - **Evidence:** Use a consistent method, repeated observations where appropriate and a table with labelled quantities and units. Keep unexpected results and investigate their cause. - **Safety:** Practical activities need teacher supervision and an appropriate risk assessment. Use the provided data or simulation where the investigation specifies it. - **Inquiry task:** State a testable question, predict the outcome using the science, then explain how your observations would support or challenge the prediction. ## Evaluate the science - Lenses are used in cameras and instruments to form useful images. - The ray model predicts paths but does not describe every wave effect or correct for all lens distortions. - **Evaluation task:** Link your conclusion to evidence, identify a limitation and suggest a specific improvement. Distinguish a measured result from an explanation of its cause.