Chapter 9: Ray Optics and Optical Instruments
NCERT Class 12 Physics, Part II, Chapter 9. 11 lessons, 22 narrated 3D videos, 15 simulations and 22 quiz questions, following the textbook section by section.
- Reflection by spherical mirrorsLook into a shiny steel spoon. On the inside you see yourself upside down; on the back you see yourself upright and small. Both sides are curved mirrors.
- The mirror equationRay diagrams tell us roughly where an image is. To get exact numbers we need an equation linking the object distance u, the image distance v and the focal length f.
- RefractionDip a pencil into a glass of water at a slant. It looks broken at the surface. Light changes direction as it crosses from one medium into another. This is refraction.
- Total internal reflectionShine a laser pointer upwards through a tank of water, towards the surface. Most of the light escapes into the air above, but a faint beam is also reflected back down.
- Refraction at a spherical surfaceA lens has two curved surfaces. To understand it, first take just one: a curved boundary between two media, like the end of a glass rod rounded into part of a sphere.
- Thin lenses and the lens maker’s formulaA thin lens is a piece of glass with two spherical surfaces, so close together that we can ignore its thickness. Light refracts twice: once going in, once coming out.
- Power and combination of lensesAn optician’s prescription does not say “focal length 40 centimetres”. It says “minus 2.5”. That number is the power of the lens.
- Refraction through a prismHere is a triangular glass prism, seen from the end. The angle between its two refracting faces is the angle of the prism, A.
- The microscopeHow big something looks depends on the angle it makes at your eye. Bring an ant closer and it looks bigger, because the angle grows.
- The telescopeThe Moon is huge, but it is so far away that it makes an angle of only about half a degree at your eye. A telescope makes that angle bigger.
- Dispersion, rainbows and scatteringThis topic is beyond the current NCERT syllabus, but it explains some beautiful sights and is useful for competitive exams. Let us start with a beam of sunlight and a…