Optical Instruments: Cameras, Telescopes, Microscopes and More

Seeing farther, smaller and safer

How do we see a galaxy billions of light-years away, a single cell in a drop of pond water, or a deer on a dark highway? Every optical device in this lesson is built from the same two ingredients you have studied: mirrors and lenses. In this lesson you will explain how cameras, telescopes, binoculars, microscopes, retroreflectors and solar ovens work, and assess their impact on society and the environment.

Cameras

A camera works much like the eye. A converging lens forms a real, inverted, smaller image on a light sensor (or, in older cameras, on film). To focus on objects at different distances, the camera moves the lens slightly closer to or farther from the sensor (the eye changes the lens shape instead). An adjustable opening called the aperture controls how much light enters, like the pupil.

Most digital cameras once used a sensor called a CCD (charge-coupled device), which turns light into electric signals pixel by pixel. The CCD was invented in 1969 by Willard Boyle, a physicist born in Amherst, Nova Scotia, and George Smith. They shared the 2009 Nobel Prize in Physics for it with Charles Kao, who was honoured for his work on optical fibres. Today many cameras, including phone cameras, use a related sensor called a CMOS sensor. CCDs are still used in many scientific and astronomical cameras.

Refracting telescopes

A refracting telescope uses two converging lenses:

• The objective lens has a long focal length. It collects light from a distant object and forms a real, inverted image at its focal point.
• The eyepiece has a short focal length. It acts as a magnifying glass, letting you view that image up close.

The image you see is inverted, which is fine for looking at stars and planets. The magnification of a telescope is:

magnification = fₒ ÷ fₑ

A large objective lens also gathers more light, so fainter objects become visible.

Reflecting telescopes

Large lenses are heavy, sag under their own weight, and bend different colours by different amounts. So most big research telescopes are reflecting telescopes: a large concave (parabolic) mirror collects the light and brings it to a focus, where a smaller mirror sends it to an eyepiece or a camera.

The James Webb Space Telescope, launched in December 2021, has a 6.5 m primary mirror made of 18 gold-coated hexagonal segments. Canada, through the Canadian Space Agency, contributed the telescope's Fine Guidance Sensor (which keeps it pointed precisely) and the NIRISS science instrument. In return, Canadian astronomers receive a share of observing time on the telescope.

Binoculars

Binoculars are two small telescopes side by side. A telescope's image is upside down, so each side contains prisms that use total internal reflection to flip the image right side up. The prisms also fold the light path back and forth, so binoculars can be much shorter than a telescope with the same magnification. Binoculars are labelled with two numbers, such as 8 × 42: 8× magnification, and objective lenses 42 mm across.

Compound microscopes

A compound microscope also uses two converging lenses, but here the object is very close:

• The objective lens has a very short focal length. The specimen is placed just outside its focal point, so it forms a real, inverted, enlarged image inside the tube.
• The eyepiece magnifies that image again. The first image sits inside the eyepiece's focal length, so you see a virtual, enlarged image.

Total magnification = objective magnification × eyepiece magnification

Retroreflectors and solar ovens

A retroreflector sends light straight back towards its source, no matter what angle it comes in at. A corner reflector uses mirrors (or, in 3-D, three surfaces) at right angles to each other: each reflection reverses part of the ray's direction. Bicycle reflectors, the reflective strips on safety vests, and many highway signs use tiny corner reflectors or glass beads so that a driver's headlights light them up brightly for the driver.

In 1969, the Apollo 11 astronauts left a panel of corner-cube retroreflectors on the Moon. Scientists on Earth still bounce laser pulses off it and time their return to measure the distance to the Moon very precisely.

A solar oven (solar cooker) uses shiny reflectors to concentrate sunlight onto a dark cooking pot. A parabolic solar cooker is a concave mirror: parallel rays from the Sun are reflected through its focal point, where the pot sits. Solar cookers need no fuel and produce no smoke, which makes them useful where firewood is scarce.

Assessing impacts (E1.2)

When you assess an optical technology, consider benefits and costs for society and the environment:

• Medicine: microscopes made it possible to see bacteria and cells, transforming the diagnosis of disease. Endoscopes (bundles of optical fibres) let doctors see inside the body without major surgery. Femtosecond lasers, which rely on chirped pulse amplification developed by Donna Strickland (born in Guelph, Ontario) and Gérard Mourou, are used in eye surgery.
• Science: telescopes changed our understanding of our place in the universe.
• Safety: retroreflectors make cyclists and pedestrians visible at night.
• Environment: solar cookers reduce the need to cut firewood and reduce indoor smoke. Satellite cameras monitor forests, ice and wildlife. But manufacturing electronics uses energy and materials, and discarded devices add to electronic waste.
• Privacy: cheap, powerful cameras and drones raise questions about privacy and surveillance.