Optical relates to sight, light and lenses. Optimal means best possible. An optical illusion; the optimal route. The c and the m are the only difference on the page; the eye and the best are the difference underneath.
The core difference
- optical — of vision or light; using light. Optical fibre, an optical illusion, optical character recognition.
- optimal — the best achievable under the circumstances. Optimal conditions, the optimal dose, far from optimal.
The tell
Optical goes with optician and optics — the eye. Optimal goes with optimum and optimise — the best. An optical solution would be one made of lenses; an optimal one is simply the best available. Anything with light in it is optical; anything with a target in it is optimal.
In use
- optical — The drawing is an optical illusion — the lines are the same length. Data travels down the optical fibre as light.
- optimal — Twenty degrees is the optimal temperature for the yeast. The schedule is not optimal, but it works.
Common slips
- an optimal illusion → an optical illusion
- the optical solution (meaning the best) → the optimal solution
Optical is the eye; optimal is the best.
Where the words come from
Optical is from Greek optikos, "of sight", from ops "eye" — the root of optician and autopsy. Optimal is from Latin optimus, "best", the superlative of bonus "good" — the root of optimism, the belief that things are for the best.
Frequently asked questions
Is optimal the same as optimum?
Optimum is the noun (the best point) and also an adjective; optimal is the adjective. "Optimal conditions" and "optimum conditions" both work.
What is an optical illusion?
An image that tricks the eye into seeing something that is not there, or misjudging what is.
Is optical related to optimism?
No — optical is Greek, from the eye; optimism is Latin, from optimus, best, like optimal.