How small can your eye see?

The idea

The limit of resolution is the smallest separation at which two close points are still seen as distinct; for the human eye at its near point of about 25 cm it is about 0.1 mm.

Two dots on paper

Draw two tiny dots on a page. Bring them closer and closer together, and keep looking from about 25 cm.

What will happen?

• At some gap the two dots will look like one
• We will always see two, however close
• The dots will vanish

Find the gap

Slide the gap smaller and find where the dots merge.

The limit of resolution

The limit of resolution is the smallest gap at which two close points still look distinct. For the human eye at its near point (about 25 cm) it is about 0.1 mm.

Most cells are far smaller than 0.1 mm, so we need a microscope to see them.

• Gap | What we see
• more than 0.1 mm | two separate dots
• 0.1 mm | just separate (the limit)
• less than 0.1 mm | one dot

Notes

The limit of resolution of the human eye at about 25 cm is about 0.1 mm: closer points look like one.

Check yourself

What is the limit of resolution of the human eye, in millimetres?

Answer: 0.1 mm

About 0.1 mm, when viewed from the near point, about 25 cm.

Two dots are 0.05 mm apart. From 25 cm, the eye sees…

Two dots are 0.3 mm apart. From 25 cm, the eye sees…

A cell is 0.01 mm across. How many times smaller is it than the limit of resolution (0.1 mm)?

Answer: 10

0.1 mm is 10 times 0.01 mm, so the cell is 10 times smaller than the smallest gap the eye can separate.

• one dot — correct. 0.05 mm is smaller than 0.1 mm, so they blur together.
• two dots. You need a gap of at least 0.1 mm.
• nothing at all. The dots are seen, but not as separate.
• two separate dots — correct. 0.3 mm is more than 0.1 mm.
• one dot. The gap is bigger than the limit.
• a blur. It is wide enough to see two.