Magnifying glass phone projector

What they will learn

• That a convex lens forms a real, inverted and enlarged image when the object is a little further away than its focal length.
• That every magnifying glass has a focal length that can be measured by projecting a window onto a sheet of paper.
• The relationship between distances and size: magnification = lens–wall distance / lens–phone distance.
• Why the image gets darker the bigger we make it (the same light spread over a larger area).

Prepare beforehand

• Measure the focal length of the magnifying glass (step 1) before buying or cutting anything: it tells you whether the box is long enough (you need at least focal length + 40 mm).
• Choose a cartoon with bright colours and a light background, download it (no notifications) and put the phone in aeroplane mode so no alerts pop up.
• On the phone: brightness at maximum, turn off auto-brightness and the screen timeout.
• Paint the inside of the box the day before or line it with black paper.
• Get the room ready: blinds down and a clear, smooth white wall (or a white sheet).

Materials and tools

• Reading magnifying glass — 1 buy
  glass or acrylic lens of Ø 75–100 mm, magnification ×2 to ×3 (focal length 80–200 mm)
• Shoebox with lid — 1
  inside about 32 × 19 × 12 cm, 12 cm high so that the centre of the magnifying glass is 6 cm above the floor, level with the centre of the screen (if it is taller or shorter, raise or lower shelf D by adding or removing layers of cardboard); if your magnifying glass has a focal length over 200 mm, look for a boot box
• Corrugated cardboard — 1 piece of 25 × 25 cm
  3 mm, for the carriage
• Black electrical tape or duct tape — 1 m
  opaque, for fixing the magnifying glass and sealing
• Matt black paint or black paper — black poster paint (≈ 50 ml) or 3 black A4 sheets
  for the inside
• Mobile phone or media player with a screen — 1
  5–6.5" screen; with rotation lock
• Modelling clay or adhesive putty — 1 ball of 2 cm
  so the phone does not slip on the shelf
• PVA glue or hot glue — a little
  for the carriage

Tools

• Craft knife and cutting mat — cutting the hole for the magnifying glass adult
• Scissors — cutting out the carriage pieces
• Ruler and tape measure — measuring the focal length and the distance to the wall
• White A4 sheet — screen for measuring the focal length of the magnifying glass
• Paintbrush — painting the inside

Pieces and templates

• A · Magnifying glass hole template ×1 — paper (for marking)
• B · Carriage: base and backrest ×1 — corrugated cardboard 3 mm
• C · Backrest brackets ×2 — corrugated cardboard 3 mm
• D · Shelf (25 mm square tube) ×1 — card 300 gsm or thin card
• E · Focusing ruler ×1 — paper

Step by step

1. Measure the focal length of the magnifying glass

   Next to a bright window, hold the magnifying glass parallel to a white A4 sheet stuck on the opposite wall and move it closer or further away until you see the window sharp and upside down on the sheet. Measure the magnifying glass–sheet distance with the ruler: that is the focal length (f). Write it down.

   Check: For a ×2–×3 magnifying glass it should come out between 80 and 200 mm. If it is more than the length of the box minus 40 mm, you need a longer box.
   Tip: Never do this with the sun: it would concentrate the heat and could burn the paper or your eyes.

2. Mark the hole

   Draw two diagonals on the outside of the short side of the box to find its centre. Stick template A with its centre on that point and draw round the circle (or draw round the lens of your magnifying glass and draw a line 2.5 mm inside it).

   Check: The circle is the same distance from the floor and from the lid.

3. Cut the hole adult

   With the craft knife, cut the circle in short sections of 1–2 cm, turning the box. Support the side of the box from inside with the cutting mat or a book so it does not cave in.

4. Paint the inside black

   Paint the inside of the box and the lid matt black (or line them with black paper). Leave to dry for 20 minutes. That way the light that does not go through the lens does not bounce around and wash out the image.

5. Fit the magnifying glass

   Push the lens of the magnifying glass in from the outside, with the rim resting on the edge of the hole. Turn the handle to one side, horizontally, and tape it to the box. Seal the whole rim with black tape so no light gets in.

   Check: The magnifying glass is straight (not tilted) and the handle does not touch the table.

6. Build the carriage

   Cut out piece B (100 × 160 mm) and fold it along the 70 mm line to form an L. Glue the two brackets C behind the backrest, one on each edge, so it stays at 90°.

   Check: With the set square or the corner of an A4 sheet, check that the backrest and base form a right angle.

7. Add the shelf

   Fold piece D along its four folds to form a 25 × 25 mm tube and glue the tab. Glue it lying flat on the base, pressed against the backrest (between the 40 mm line and the fold).

8. Stick on the focusing ruler

   Stick ruler E on the floor of the box, lengthways and next to one side, with the 0 touching the magnifying glass wall. It will tell you how far away the phone is.

9. Get the phone ready adult

   Play the video full screen in landscape, brightness at 100 %, and rotate the phone 180° (upside down). Turn on rotation lock so the image stays rotated and does not flip back by itself.

   Tip: If the phone rotates the image back again, lock the rotation with the phone the normal way up, start the video and then turn the phone round.

10. Place the phone on the carriage

    Rest the phone in landscape on the shelf, with the screen facing the magnifying glass and leaning on the backrest. Stick a small ball of modelling clay behind it so it does not slip. Put the carriage in the box with the screen at a distance of f + 10 mm from the magnifying glass (check the ruler).

    Check: Looking through the magnifying glass from outside, the centre of the screen is in the centre of the lens.

11. Aim at the wall

    Place the box on a table 1–1.5 m from a white wall, with the magnifying glass facing the wall and perpendicular to it. Turn off all the lights and lower the blinds.

12. Focus

    With the lid off, slide the carriage very slowly towards the magnifying glass or backwards, 2 mm at a time, until the image on the wall looks sharp. Note the mark on the ruler. Put the lid on without moving the carriage.

    Check: Letters or the edges of the drawings should look crisp. For a wall at 1 m and f = 100 mm, the phone ends up about 110 mm from the magnifying glass and the image is about 120 cm wide.

Does it work? The test

In a completely dark room, project a video 1 m from the wall, focus and measure the width of the image with the tape measure.

What should happen: With a magnifying glass of f = 100 mm and the wall at 1 m, the phone ends up at ≈ 110 mm and the image is about 9 times bigger than the screen (a 14 cm wide screen gives ≈ 1.2 m), the right way up, sharp in the centre and slightly blurry with colour fringes at the edges. It looks dim: you need total darkness.

If something goes wrong

• There is no way to get it in focus — The box is shorter than the distance needed (long focal length) or the phone is closer than the focal length. Solution: Measure the focal length again. The phone must be between f and 1.3·f. If f is greater than the length of the box minus 40 mm, use a longer box or extend the box with a black cardboard tube at the front.
• The image is cut off or in a corner of the wall — The centre of the screen is not lined up with the centre of the magnifying glass. Solution: Raise or lower the phone with more or less shelf (add layers of cardboard) and centre it across the width using the line on the backrest.
• The image is upside down — The lens inverts the image and the phone has not been rotated 180°, or the screen has rotated back by itself. Solution: Rotate the phone 180° and turn on rotation lock.
• It looks very dark or washed out — Light in the room, low brightness or an image that is too big. Solution: Total darkness, brightness at 100 % and move the box closer to the wall (0.7–1 m): the image will be smaller but much brighter. Use videos with bright colours.
• Sharp in the centre and blurry at the edges — Aberrations of a simple magnifying glass lens, or the screen not perpendicular to the axis. Solution: This is partly normal. Check with the brackets that the phone is straight; sticking a black card ring with a 50 mm opening in front of the magnifying glass improves sharpness (with a little less light).
• The image shakes or goes out of focus by itself — The carriage is sliding. Solution: Once it is in focus, fix the base of the carriage to the floor of the box with a piece of tape.

Safety

• Never use the magnifying glass in the sun or leave it in the sun next to the box or by a window: it concentrates the rays and can burn cardboard, paper or skin in seconds.
• Do not look directly at the sun through the magnifying glass.
• The craft knife is used by an adult with a cutting mat; the child marks and holds the box.
• Glass magnifying glasses can break: if one falls, an adult should pick it up.
• Do not leave the phone charging inside the closed box for a long time: it heats up. Sessions of 20–30 minutes, then take it out to cool.

The science behind it

For children

A magnifying glass is a piece of glass that is fatter in the middle than at the edges, and that makes the rays of light bend and come together. The light from each tiny dot on the phone screen goes through the magnifying glass and comes together again at a point on the wall, except that everything crosses over on the way and the image arrives upside down. That is why we turn the phone round at the start: so it arrives the right way up. Because the rays spread out a lot on the way to the wall, the image becomes huge, but the light is spread out and looks fainter; that is why you have to turn off all the lights.

For the adult

The magnifying glass is a converging lens with focal length f. According to the thin lens equation 1/f = 1/u + 1/v, if the object (the screen) is at a distance u slightly greater than f, a real, inverted image forms at v: for f = 100 mm and v = 1000 mm, u = 111 mm; for v = 1500 mm, u = 107 mm. The lateral magnification is m = v/u ≈ 9 and 14 respectively. That is why focusing is so sensitive (a few millimetres) and you need a carriage rather than modelling clay. The image is rotated 180° (inverted on both axes), not mirrored, so you only need to rotate the phone 180° to see it correctly from behind the projector. Brightness: a screen of ≈ 500 cd/m² and 140 × 65 mm seen from a Ø 90 mm magnifying glass at 110 mm (solid angle ≈ 0.5 sr) sends about 2–3 lumens to the wall; spread over 1.2 × 0.55 m this gives ≈ 3–4 lux, visible only in total darkness (illuminance falls with the square of the magnification). A simple lens has spherical and chromatic aberration: blurry edges with colour fringes; a diaphragm reduces the aberration at the cost of light. The black inside absorbs the light that does not go to the lens and prevents haze.

• converging lens: a lens that is thicker in the middle and brings rays of light together (the magnifying glass).
• focal length: the distance from the lens to the point where it brings together the light from something very far away.
• real image: an image that can be caught on a wall or a sheet of paper.
• magnification: how many times bigger the image is than the object.
• focus: to move the lens or the object until the image looks sharp.

By age

• 5–7 years: The adult measures the focal length, cuts the hole and sets up the phone. The child paints the inside, assembles the carriage with tape and slides the carriage to focus while watching the wall: “is it sharp yet?”.
• 8–10 years: They do all the steps except the craft knife cutting. Let them measure the focal length themselves with the window and check that the image is rotated.
• 11–12 years: Challenge: test the lens equation. Measure u and v at three distances, calculate 1/u + 1/v and compare it with 1/f. Calculate the magnification v/u and compare it with the one measured on the wall.

In a group or in class

Teams of 3–4 (6–7 projectors for 25 children). Each team brings a shoebox; the teacher provides identical ×2–×3 magnifying glasses (≈ €5 each) and checks their focal length beforehand. 60-min session: 10 min measuring the focal length with the window (everyone at once), 30 min assembly, 20 min projecting in the darkened classroom (take turns projecting so the light from one screen does not drown out another). Use a short shared video on the teacher's phone if the children do not have one.

Challenges

• Add a black cardboard tube between the magnifying glass and the wall of the box so you can use magnifying glasses with a long focal length.
• Make a focusing mechanism you can work from outside: a wooden skewer that goes through the back of the box and pushes the carriage.
• Project with two magnifying glasses together (pressed against each other): what happens to the focal length?
• Design a projection screen with a large box lined with white paper and compare it with the wall.

More free projects

• A rainbow from a glass and a slit
• Single-drop microscope
• Spinning colour disc
• Constellation projector
• Dark box that projects images
• Halloween shadow lantern
• Hologram pyramid
• Mirror and colour tube
• Mirror periscope
• Light spectrum viewing box

See all projects