# Double Slit Experiment

How light and single electrons build up interference stripes through two slits, and why the stripes vanish once the path is recorded\.

By PETE\!

Topic: Physics & Chemistry

[Open the interactive Jig](https://ask.jig.com/j/8LyEhMJZGP7A)

## Transcript

### 1. Two slits, one beam

A red laser shines on a black plate with two narrow slits\. On the white screen beyond, the light does not make two bright lines\. It makes a row of stripes\. By the end you will see why, and why it puzzles physicists\.

Drag empty space to turn the bench; pinch or scroll to zoom\.

### 2. Cover one slit

The brass shutter covers the second slit\. Light squeezing through one narrow slit spreads out into a single broad glow\. Click the shutter, or use the switch, to open the second slit and watch stripes appear\.

Spreading after a narrow gap is called diffraction\.

### 3. Waves from two slits

Each slit becomes a new source of waves\. Where a crest from one slit meets a crest from the other, they add up: bright\. Where a crest meets a trough, they cancel: dark\. Those bright lanes fan out toward the screen\.

Not to scale: real slits are a quarter of a millimetre apart\.

### 4. Path difference

The first bright stripe beside the centre sits where the path from the far slit is exactly one wavelength longer\. Move the slits closer together and the stripes spread further apart\.

Stripe spacing = wavelength x distance to screen / slit spacing\.

### 5. Colour changes the spacing

Shorter waves need less distance to fall one wavelength behind\. Switch from red to green or violet light and the stripes squeeze closer together\.

Red 650 nm, green 532 nm, violet 405 nm\.

### 6. One electron at a time

Now the source fires electrons, so few that only one is in the apparatus at a time\. Each one lands as a single dot\. Fire them and watch: at first the dots look random, then stripes build up\.

Tonomura and colleagues, 1989: 10, 200, 6,000, 40,000 and 140,000 electrons\.

### 7. Which slit did it go through?

Small sensors behind the slits can record which way each electron went\. Switch them on and the stripes vanish, leaving two overlapping bands\. Recording the path removes the interference\.

No conscious observer is needed: any record of the path is enough\.

### 8. The heart of quantum mechanics

Light and electrons travel like waves through both slits, yet arrive as single particles\. Richard Feynman said this experiment holds the heart of quantum mechanics, and it is still the clearest view of it\.

Molecules of up to 2,000 atoms have shown the same stripes \(2019\)\.