Trapped Light

Why it matters

Submarine periscopes don't use mirrors. Mirrors fade and ghost. They use plain glass prisms — and a trick the glass does for free.

The idea in plain words

Why engineers pick prisms over mirrors. Tap the terms.

45° prism: δ = 90° or 180°

A periscope uses two 45° prisms. Trace the turns.

Two lossless right-angle turns — a mirror-free periscope

• application: 90° turn — one TIR at the hypotenuse — periscopes (two prisms, two turns)
• application: 180° turn — two TIRs — binoculars fold the light path, bicycle reflectors send light home
• application: erecting — prisms can also flip an inverted image upright without deviation
• Prism 1: light enters, TIRs at 45°, turns 90° downward
• Prism 2: TIRs again, turns 90° toward the eye

Predict first

Why does a 45°–45°–90° glass prism reflect light like a perfect mirror?

45° > C(glass) ≈ 41.8°, so 100% of the light reflects off the inside of the hypotenuse — no coating, no loss, no ghost image.

• Light hits the inner face at 45°, beyond glass's critical angle (41.8°) — total internal reflection — correct
• The glass is silvered at the back
• The prism disperses light back

What you do

You're in the submarine, the ship is up on the surface. Rotate each prism until its 45° face catches the light — two perfect 90° turns and the ship lands in your eye. Wrong tilt and the beam leaks to the sky or bounces away.

Check yourself

Prisms beat mirrors in periscopes because…

Silvered mirrors absorb a little light and give faint double reflections; TIR is total and permanent.

An optical fibre keeps light inside for kilometres using…

Light strikes the core–cladding boundary beyond C thousands of times, never escaping — your internet rides on TIR.

A diamond sparkles more than a glass copy of the same cut because…

C(diamond) ≈ 24° vs C(glass) ≈ 42°: most rays inside a diamond TIR many times before finding an exit — that's the fire.

• TIR reflects 100% of the light and never tarnishes — correct
• prisms are lighter than mirrors
• prisms magnify the view
• repeated total internal reflection along the fibre wall — correct
• a vacuum core
• electric current in the glass
• its critical angle is much smaller, so far more light gets trapped and bounced — correct
• diamond is more transparent
• diamond is colourless