E. The Wave Nature of Light · The Wave Nature of Light

Light-Wave Technologies and Their Impact on Society

E1.1 E1.2 E2.1 E3.2 A1.10 A2.1About 75 minutes

Learning goals

I can analyse how a technology uses the wave properties of light (interference, diffraction, polarization).
I can explain how holograms, optical discs, anti-reflective coatings, spectroscopes, polarizing lenses and night-vision devices work.
I can assess the social and environmental benefits and costs of light-wave technologies.
I can name careers that use the wave nature of light.

Words to know

hologram
A recording of the interference pattern between light from an object and a reference beam; when lit, it diffracts light to recreate a three-dimensional image.

reference beam
In holography, the part of the laser light that goes straight to the film without touching the object.

pit and land
The low and high areas along the track of an optical disc; their edges encode data.

risk–benefit analysis
A careful comparison of the advantages and the possible harms of a technology, including who is affected.

image intensifier
A night-vision device that amplifies the small amount of visible and near-infrared light present.

thermal imaging
Forming images from the infrared radiation that warm objects emit.

Physics you carry in your pocket

Your phone screen uses polarizers. Its camera lens has an anti-reflective coating. Its data arrives through radio waves and, somewhere along the way, through optical fibre. The hologram on a bank card or a passport is a recorded interference pattern. Each of these technologies uses the wave nature of light, and each has effects on people and the environment. This lesson brings the unit together: first how the technologies work, then whether they are good for society, and for whom.

How to analyse a technology

Purpose: what problem does it solve?
Physics: which wave property does it use (interference, diffraction, polarization, refraction)? Explain with a diagram and, where possible, a calculation.
Benefits: to individuals, communities, the economy, health, science.
Costs and risks: health, safety, privacy, cost, fairness of access, environmental effects (materials, energy, waste).
Who is affected, and who decides?
Your judgment, with reasons, and one improvement or alternative.

Holograms

A photograph records only how bright the light from each point is. A hologram also records the phase of the light waves, which is what carries depth.

A laser beam is split in two. The reference beam goes straight to a photographic film. The object beam lights the object, which scatters light onto the same film.
The two beams are coherent (same laser), so where they meet they make a stable interference pattern of extremely fine fringes. The film records this pattern.
Later, when light shines on the developed hologram, the fringe pattern acts like a very complicated diffraction grating. It diffracts the light into waves just like those that once came from the object, so your eyes see a three-dimensional image that changes as you move your head.

Uses include security features (many bank cards, ID cards and passports use holograms because they are hard to copy), art and museum displays, and data storage research.

Making and viewing a hologram (schematic). Left: a reference beam and light scattered from the object interfere on the film, which records the fringe pattern. Right: light shining on the hologram is diffracted by that pattern into waves that seem to come from the object, so you see a 3-D image where the object used to be.

Optical discs: reading data with interference

An optical disc (CD, DVD, Blu-ray) stores data as a spiral track of pits and flat lands, read by a laser from below. The pits are made about one-quarter of the laser's wavelength (in the plastic) deep. When the laser spot sits partly over a pit edge, light reflected from the pit and light reflected from the surrounding land differ in path by about 2 × λ/4 = λ/2, so they interfere destructively and the detector sees a dip in brightness. Edges between pits and lands therefore show up as changes in the signal, which the player decodes as ones and zeros. The spiral track itself is a diffraction grating, which is why discs show rainbow colours.

Diffraction also limits how small the laser spot can be (lesson on resolution), which is why each generation used a shorter wavelength.

Reading a disc (schematic, not to scale). Over flat land, the laser reflects strongly. Over a pit edge, light from the pit and light from the land differ by half a wavelength and cancel, so the reflected signal drops.

Worked example

Example 1: How deep is a CD pit?

A CD is read with a 780 nm infrared laser through polycarbonate plastic (n = 1.55). Find the pit depth that gives destructive interference between light from the pit and from the land.

Wavelength in the plastic: λₙ = 780 ÷ 1.55 = 503 nm.
The light reflected from the land travels to the land and back, an extra 2 × depth compared with light from the pit (or the reverse, depending on which side you measure from). For cancellation: 2 × depth = λₙ/2.
Depth = λₙ/4 = 503 ÷ 4 = 126 nm.

About 126 nm, roughly a thousandth of the thickness of a sheet of paper.

Worked example

Example 2: Anti-reflective coatings on solar panels

For light hitting glass (n = 1.52) straight on, the fraction reflected at one surface is R = ((n − 1)/(n + 1))². Find R for uncoated glass and explain why panel makers add anti-reflective coatings.

R = ((1.52 − 1)/(1.52 + 1))² = (0.52/2.52)².
R = (0.206)² = 0.043, so about 4.3% of the light is reflected at the front surface.
A quarter-wave coating makes the two reflections cancel for wavelengths near the middle of the visible spectrum, reducing this loss substantially.

About 4% of sunlight is lost to reflection at each uncoated glass surface. An anti-reflective coating sends much of that light into the cells instead, so the same panel produces more electrical energy: a benefit for both cost and the environment.

Profiles of more technologies

Spectroscopes and spectroscopy. Gratings separate light into wavelengths so substances can be identified by their spectral lines. Uses: identifying elements in stars, checking for pollutants in air and water, quality control in food and medicine, forensic science. Impact: mostly beneficial; it helps monitor the environment without collecting samples from dangerous places.

Polarizing lenses. Sunglasses with a vertical transmission axis block horizontally polarized glare from water, snow and roads. Benefits: comfort and safety for drivers, people who fish, skiers. Drawbacks: they can make some LCD screens (phones, some dashboard displays) hard to read at certain angles, and they do not protect against looking at the Sun.

Xenon (HID) headlights. These gas-discharge lamps make an intense light with a bluish tint, using an emission spectrum rather than a glowing filament. Benefits: brighter, more efficient lighting. Concerns: glare for oncoming drivers if headlights are poorly aimed.

Night-vision devices. Image intensifiers amplify the tiny amount of light present; thermal imagers form pictures from infrared radiation emitted by warm objects. Benefits: search and rescue, wildlife research, firefighting (seeing through smoke), security. Concerns: privacy, military use, and cost.

Wireless networks. Wi-Fi and cellular networks carry data on electromagnetic waves produced by oscillating charges in antennas; antennas are designed using wavelength, diffraction and polarization. Benefits: access to information, education, health care and emergency services, including in rural, remote and northern communities, many of which still have slower or less reliable service than cities. Concerns: electronic waste, energy use of data centres, and public questions about health; in Canada, Health Canada sets exposure limits for radio-frequency energy in its guideline called Safety Code 6.

Technology	Wave property used	One benefit	One concern
Hologram	interference and diffraction	hard-to-copy security features	counterfeiters keep improving
Optical disc	interference, diffraction	cheap, durable data storage	plastic waste; replaced by streaming
Anti-reflective coating	thin-film interference	more light into lenses and solar panels	coating materials and manufacturing energy
Spectroscope	diffraction grating	identifies substances from a distance	few: mainly cost of precise instruments
Polarizing lenses	polarization	less glare, safer driving	LCD screens can be hard to read
Night vision	detecting light and infrared	search and rescue	privacy
Wireless networks	EM waves from oscillating charges	access to services	e-waste, energy use

A starting point for analysis. Each row could be argued in more depth.

Worked example

Example 3: A modelled risk–benefit judgment

Should a town require anti-reflective coatings on the glass of a new public building's large solar array? Model a short analysis.

Physics: a quarter-wave coating makes reflections from its two surfaces cancel near the middle of the visible spectrum, so more light enters the cells.
Benefits: more electricity from the same panels and roof area; less glare reflected toward neighbours and drivers.
Costs: higher purchase price; coatings can wear over time; manufacturing uses energy and materials.
Who is affected: taxpayers (cost), neighbours and drivers (glare), the community (cleaner electricity).
Judgment: most anti-reflective coatings add a small cost compared with the panels themselves, and they reduce glare too, so requiring them is reasonable, if the town checks how long the coating lasts.

A strong answer uses the physics, weighs benefits against costs for different groups, and reaches a reasoned, conditional conclusion.

Careers that use the wave nature of light

Optical and photonics engineers design lenses, coatings, lasers, fibre-optic networks and sensors.
Astronomers and astrophysicists use gratings and interferometry to study stars and galaxies.
Optometrists and ophthalmologists test vision and prescribe lenses with coatings and polarizing or tinted filters.
Telecommunications technicians and engineers build wireless and fibre networks.
Forensic and analytical chemists identify substances with spectroscopy.
Materials scientists design thin films and photonic structures.

Most of these careers begin with university programs in physics, engineering, chemistry or health sciences, which is where this Grade 12 course leads.

Just for fun

A hologram, a polarizer and a diffraction grating form a band. The hologram handles the 3-D stage effects, the polarizer only plays in one direction, and the grating insists on splitting every chord into all its colours.

Hands-on activity

Light-wave technology audit and report

You need: a notebook, and access to everyday items (sunglasses, eyeglasses, a camera or phone, an old CD or DVD, a bank card or ID card with a hologram).

Find them: walk through your home and list every item that uses the wave nature of light. Aim for at least six. For each, name the wave property it uses.
Look closely: tilt a card with a hologram and watch the image change; look at the reflection on eyeglasses or a camera lens for the faint coloured tint of a coating; tilt a disc under a lamp.
Choose one technology and write a one-page analysis using the six steps in the tip box. Include a labelled diagram and at least one calculation from this unit (for example, a coating thickness, a pit depth or a grating angle).
Finish with a judgment: overall, is this technology good for society and the environment? Who benefits most, and who bears the costs? Suggest one improvement.

What to look for: a good audit finds items such as polarized sunglasses, coated lenses, LCD screens, discs, holograms, Wi-Fi routers, and fibre internet equipment. A strong report explains the physics correctly, uses evidence for benefits and costs, considers several groups of people and the environment, and reaches a clear, reasoned conclusion.

Your turn

What does a hologram record that an ordinary photograph does not?

Why must holograms be made with laser light?

How deep should the pits on a DVD be if it is read at 650 nm through plastic with n = 1.55?

Name the wave property used by polarized sunglasses and by anti-reflective coatings.

Give one benefit and one concern of night-vision technology.

What does Health Canada's Safety Code 6 deal with?

Name two careers that use the wave nature of light.

More practice: warm-up

What acts as a diffraction grating on a CD?

In a hologram, what are the two beams called?

What does a spectroscope separate light into?

What kind of radiation do thermal imagers detect?

Why do optical discs show rainbow colours?

More practice: on your own

Find the pit depth for a Blu-ray disc read at 405 nm through plastic with n = 1.55.

Use R = ((n − 1)/(n + 1))² to find the fraction of light reflected at a water surface (n = 1.33) for light arriving straight on.

Why are holograms useful as security features on cards and passports?

Describe one environmental cost of optical discs and one way to reduce it.

Explain how wireless networks can affect fairness of access to services in Canada.

Challenge

A student says, "Polarized sunglasses block all glare, so they are always safer for driving." Evaluate this claim.

Explain why light from an ordinary light bulb cannot be used to make a hologram of a large object, using the idea of coherence.

Write a two-sentence argument for or against replacing a school's light-microscope set with cheaper digital microscopes that give larger on-screen images but use the same small lenses. Use the idea of resolution.

Check your understanding

Quiz

A hologram can show a three-dimensional image because it records…

Optical disc pits are about one-quarter wavelength deep so that…

Which technology mainly uses thin-film interference?

Which is a valid concern about night-vision and thermal-imaging devices?

In Canada, limits on exposure to radio-frequency energy from wireless devices are set in…

The best way to judge whether a light-wave technology benefits society is to…

For parents and teachers

Big idea: expectations E1.1 (analyse a technology that uses the wave nature of light) and E1.2 (assess impacts on society and the environment), with careers (A2.1). There is no single right opinion; marks come from correct physics, evidence and balanced reasoning.

How to help: do the home audit together and talk about the questions in the tip box. Push for reasons: "Who benefits? Who pays? What happens when it is thrown away?"

Common difficulties:

Describing what a technology does without explaining the physics (which wave property, and how).
One-sided judgments. Encourage your teen to argue the other side first.
Unsupported health claims about wireless devices. Point to reliable sources such as Health Canada.