Lemon juice is very effective in lightening the dark spots caused by acne or pimples. First, cleanse the scar with water. Dab one teaspoon of lemon juice into a cotton ball and smooth it over the acne marks. Leave for ten minutes, then rinse. Use caution with lemon juice because it can make your skin photosensitive. So be certain to use a sunscreen on any area you treat with lemon juice prior to going out into the sun.